It seems like we're getting not just people who played CoH years ago but some new folks putting on the cape. Welcome!
One of the first, and most important things, to know about City of Heroes is that a lot of the balance numbers in the game...aren't. Meaning in another game you might find a BIG damage buff being +15%. In CoH that's less than the buff one of the minor starter attacks gives.
Q: Why's that so important?
Simple, with buffs and debuffs being so incredibly powerful in the game, it means that "healing" does not have the same spot that it does in a lot of other MMOs.
If someone is "Looking for Healer" they are asking for the wrong thing. To put it even more clearly: YOU DO NOT NEED HEALING.
What you need, or rather what all teams benefit from, is support, and Defenders have some of the best support options in the game. (As a side note: no teams actually need support - back when the game was live all sorts of crazy teams did all sorts of crazy things, from 8x blasters taking on the Statesman TF to 8x defender teams absolutely trivializing Giant Monster encounters - in CoH, you can make almost anything work if you know what you're doing).
Q: Ok, so what's the BEST Defender then?
The ones that you have on your team.
Q: That's not helpful. What should I play?
Whatever you want!
Q: Still not helpful! Why should I play any of these!
Ok, now there's a question we can answer.
One reason to pick a powerset is "because I haven't played it before". Another is "because it fits this concept that I have". What's good to know in either case is what the set excels at and what it's going to be leaning on in the long run and at max level.
Defender Primary Sets and Where They Excel
Cold's versatile. If provided some solid defense buffs which prevent damage by making attacks miss more often. For targets without native defense, this will noticeably decrease the damage they take and for targets with a fair bit of defense to start with it will put them in demi-god mode.
Defense has a few weaknesses though, notably that attacks can still get lucky, and some mobs have attacks which also reduce defense, and when I say reduce, I don't mean by a little.
Fortunately, Cold brings more to the table than that. Their +Max HP power helps take the edge off the hits that do get through, their slows mean mobs have less chances to sneak a lucky attack in and their later powers absolutely wreck mobs in terms of reducing the damage they do, reducing their resistance to damage (which massively amplifies the damage they take, and reducing their regeneration (which is critical in fighting high level arch-villains)
Dark's got a nice grab bag of tricks too, with a heavy theme of reducing a mobs accuracy on the side. -Acc powers are great against most mobs, since they're effectively defense for everyone, but some mobs, like AVs are very resistant to the effect and will still hit you just fine.
Dark has one of the better heals in the game in terms of raw strength, but it's tied to a to hit check, so can be problematic on some highly defensive mobs (or when the Dark def themselves are debuffed).
Another noteworthy trick they have is that Dark is the one powerset in the game which gets a multi-target Ressurect power. Meaning you target one mob and ressurect everyone who's dead nearby. This can absolutely turn around a team fight that's going south and is invaluable for some content.
Apart from that they have a variety of control powers, which are nice but not best-in-class. They also get a pet which is unusual for defenders.
I'm an Empathy main. My first 50 on live was an Empath and my first 50 here will be one too. I love Empathy.
But Empathy Defenders are not the best.
They are what the clueless will ask for, they are what people from other games think all Defenders are, but they are not the best defenders. Honestly I wouldn't even put them in the top half of the list.
Why? Well they have some of the strongest and most reliable heals, but in CoH avoiding and preventing damage is exponentially more powerful than trying to heal it.
That said, Empathy is still a fun and powerful set. It's heals are close to top tier and are absolutely the most reliable ones around. No to hit checks, no mobs needed. Fire it off and the target will be healed. End of story.
Empathy's most important powers aren't it's heals though. With Fortitude, you can increase the Defense, Damage, and To Hit of other heroes, boosting them well beyond their normal limits. With Clear Mind you can ward off even the strongest mezzes from people who are otherwise helpless against crowd control. With Recovery Aura you can offer the team bursts of near inexhaustible Endurance, and Adrenalin Boost is just absurd in terms of the buffs it provides.
A classic of the super hero genre, Force Field brings a whole lot of defense to the table.
Of all the Defender sets, FF is probably one of the weaker ones, but to give a sense of what that means, with just it's bubble powers, FF can reduce a mobs chance to hit to less than half it's normal rate. And that's for heroes without any defense. Stack bubbles on someone with defense already and the chance to hit them plummets even further.
Force Field is held back by a lot of their powers causing Knockback, which is generally inconvenient, but something that can be fun and can be worked around to some extent. It's also held back because it doesn't provide much in terms of critical debuffs for hard targets, which is where a lot of other defenders shine.
Okay, I lied. There is a best Defender set. It's Kinetics. Fight me.
I might possibly be biased here, I'll admit, but Kinetics is my drug of choice. It's powers are just fantastic for letting you go Go GO!!! From Transfusion which is one of the strongest heals in the game, to Siphon Power which gives a nice damage buff, to Siphon Speed so you go faster and they go slower, to Increased Density which grants mezz protection and Knockback protection (one of the few methods of getting that in the game if your powerset doesn't supply it), to the glory that is Speed Boost*, the endless Endurance which Transference brings and the absolutely absurd "Say Hello to the Damage Cap" that is Fulcrum Shift.
Basically, if you need a buff, Kinetics has it, and will give you more of it than you can imagine.
*Technically this can be a problem with Speed Boost, where the constantly-on Super Speed it gives you can be more than some players can handle driving around with. Fortunately the devs added a method of declining the movement speed buff portion of the power, and keeping the Endurance Recovery buff which means it is firmly in the god tier of buffs where it belongs.
One of the later additions to the game, Nature is a balanced set that offers a wide array of options. Nature's claim to fame is having a well balanced helping of almost everything that you'd want. It's not best-in-class for any one buff or debuff, but it makes a solid showing in a lot of important ones like -Resistance, -Damage, -Regen.
In a sense, Nature is Empathy's more well rounded sibling and tends to play better with other defenders since it can augment a lot of different niches like a generalist should.
If someone did something stupidly difficult in City of Heroes when the servers were live, chances are they were using Radiation Emission.
Rad has a few drawbacks to it. It's vulnerable to idiots teammates who don't understand how to keep mobs grouped up. The Rad defender needs to pay attention or their target may run off with a debuff and drag in a lot more aggro than is desired. And the Rad isn't mezz proof and a mezzed Rad is one whose powers all shutoff.
Why in Statesman's name would you want to play such a character?
Because their powers are so damn strong that it's worth it.
Rad defenders have a heal. If they ever need to use it, they are doing something wrong.
Between Rad Infection and Enervating Field, a Rad Defender can turn spawns into helpless kittens. Themobs can't hit you, they can't hurt you, they have no defense so you don't miss them, and they have no resistance so you do a stupid amount of damage to them.
The devs made Rad before they understood how math worked I would guess, and then regretted it ever after.
Oh, but wait, there's more! Lingering Radiation is so strong that it shuts off even the most powerful AVs regeneration, Fallout turns a dead body into a deadly weapon (granted this is mostly a toy), and Mutation is a better rez than what Empathy gets since you come back stronger than when you died, and then there's Accelerate Metabolism, which buffs Recharge, Recovery, Damage and Resistance to Mezzing.
I said Kinetics got the best buffs? Accelerate Metabolism is climbing into the ring with a real mean look in its eyes and I'm kind of afraid.
Force Field bubbles give defense and make you harder to hit. Sonic Resonance bubbles give Damage Resistance and let you take less damage when you are hit. Put them both together and watch mobs regret every life choice they ever made.
But Sonic is a lot more than the Resistance bubbles. Sonic also offers -Resistance on Foes, which is critical for defeating tough opponents since it's effectively a damage buff for everyone, even people at the damage buff cap.
Sonic gets some other tricks as well, but the shields and the -Res are the big ticket items they bring to a party.
Because Rad defenders need someone to hate.
Storm is a really potent powerset, but it can play really poorly with others in the wrong hands. This is because Storm has a lot of Knockback, which tends to scatter mobs, which in turn means AoEs do a lot less damage and people like Rad defenders who have area based debuffs get really grumpy.
Smart Storm users can use their Knockback to push people *into* the debuffs and AoEs, but its pretty fun to just go full on Chaos mode and wreck stuff without worrying about that.
Big powers for Storm include Snow Storm (which slows mobs in a wide area and helps keep them together), Steamy Mist (which grants Stealth to everyone party member nearby), Freezing Rain (more slows + a very useful -Def and -Res for mobs in a wide area), Hurricane (for a -ToHit deubff as strong as Rad's), and Lightning Storm (for just random zappy fun).
Or, as it was originally setup, "the Villain's version of Empathy".
Thermal's got a similar healing focus to Empathy, but it's weaker at that in exchange for some highly useful buffs and debuffs in. Buffs include Thermal and Plasma Shield for +Dmg Resistance (like Sonic) and Thaw (which is similar to Clear Mind), and Forge (which is similar to Fortitude but with more damage and no defense).
With Heat Exhaustion and Melt Armor they've got access to debuffs for Resistance, Regeneration and Damage...except that the cooldown is so long that those can't be made permanent like a Rad defenders are out of the box. Still good mind you, just not best in class.
Time Manipulation has a variety of good powers, though some of the best suffer the same issue of having significantly longer cool downs than their duration.
Still there's some good stuff here. Chrono Shift gives everyone Recharge and Recovery, Slowed Response inflicts -Def and -Res, Temporal Selection is Time's version of Fortitude but arguably better since it gives +Damage, +Recharge and +Regen and Time's Juncture is like a weaker version of the Radiation clouds except it's centered on the Time defender so it can't run away and aggro other groups (unless the defender wants to do that).
With no direct healing, a quick view of traps would suggest that it offers little defensively to the team. In practice though a well played (and worked with) Traps defender can keep a team very safe and rolling along quite well. Web Grenade and Caltrops provide some indirect damage reduction since they keep the mobs out of melee. Triage Beacon is a strong +Regen, Acid Mortar offers -Def and -Res, Forcefield Generator offers +Defense in a wide area, Posion Trap offers as much-Regen as Rad's strongest power, Seeker drones give -ToHit and -Dmg and Trip Mine and Time bomb both deal quite a lot of damage.
In terms of tool kits, Traps has it all, but it does take a while to mature and it does require thoughtful play in terms of where things are places and when they're used.
The original answer to "Defenders are not healers". Trick Arrow is all about debuffs and offers essentially nothing in terms of defensive abilities or heals. That's not a problem at all, though Trick Arrow does lag a bit overall since it's best debuffs come later on and it doesn't include -Regen which many of the other debuffing sets supply.
That said, Poison Gas Arrow, Acid Arrow and Disruption Arrow will certainly help with wrecking a mobs day and Oil Slick Arrow is both good fun and one of the few abilities that can combo with another player (anyone with a fire attack can light it on fire to do damage in a wide area).
Q: Wow that's a lot to pick from...and a lot to read...what powerset should I pick again?
All of them. This is City of Heroes, we've got 1,000 slots per server, use 'em!
Q: What About the Secondary powers?
Seriously? This wasn't long enough?
Ok, short notes on the secondaries.
First: Take them. Use Them. Even if you're an Empathy heal bot. Your damage is a lot less than a serious damage dealer but its still something and with all the buffs and debuffs flying around its still super useful to have.
Archery: Lethal damage mostly, overall good, Rain of Arrows is excellent, downside is no secondary effect to speak of.
Assault Rifle: Honestly a bit disappointing, Flame Thrower is good, Full Auto is also good, downside is it hits a bit weak and doesn't have a good attack chain. Was unhappy with it on live, but some people loved it. Decide for yourself if it fits your concept.
Beam Rifle: New set, -Res and -Def sound interesting and defenders are all about the side effects. Would suggest this over Assault Rifle, but don't have the experience to back it up.
Dark Blast: -To hit as a secondary is nice and the Tentacles are a great immob for keeping mobs in place.
Dual Pistols: So you want to be a Gun-fu fighter? Here ya go. AoEs are good, secondary effects include -Def and -Res which is nice.
Electrical Blast: A bit disappointing. Damage is somewhat low and the secondary effect of End drain is a trap of uselessness without a build designed around it.
Energy Blast: Pretty and fun, but the secondary is Knockback which can be a great evil in the wrong hands and does little to help with hard content.
Fire Blast: Would you like some damage to go with your damage? Then fire is for you. Of course, defenders don't get the damage Blasters do so it'll be a bit less impressive and there's no secondary to benefit from the defender's higher values.
Ice Blast: Like Fire but substitute "slows" for extra damage. Upside, defender slows are as good as they get.
Psychic Blast: Psychic damage isn't widely resisted but it's also on the low end, so that's nice, also has a good AoE and mezz options.
Radiation Blast: Rad gets all the good toys, including solid attacks which do -Def and make hitting easier for everyone.
Sonic Attack: The attacks don't have the best damage, but they inflict -Res which means everyone (including you) hit for more. Also a good sleep in Siren's Song.
Water Blast: Some neat mechanics and some decent debuffs.
Hope this all helps!