For single target, Ninja's are far and away the best for damage. They do decent in groups due to how the AI responds to caltrops, too. They are a bit on the endurance heavy side though, and obviously the suckers are squishy but their low cooldown, constant damage ((With Jonin dealing high damage to boot, second only to Skeletons but without Skeletons flaws)) and the Oni's damage is non-physical and decent stacks up real quick. If you can keep them alive they'll murder anything in a hurry. Even if you just meatbag them they'll still do a good job in making things dead.
For multi-target its between Bots and Demons. From Bots it primarily comes from the fairly short cooldown on the Assault Bots missiles, with some help being tossed in from Protectors Photon Grenade. Little bots technically have a conal attack but the chances of it hitting more then one or two enemies is meh at best. Demons are in a similar position, but with the breath attacks from the demonlings being a bit better in AoE situations. Hellfire Gargoyle and his constant AoE damage coupled with his AoE abilities make him the star of that show, with the demon Prince's AoE focusing more on crowd control and debuffs.
Pure survivability, that's going to the Bots obviously. Don't even think I need to explain this one. I will say that Bots/Kinetics is weird. Bots is knockback heavy and Kinetics heal focuses on the target, which is usually flying across the room so that makes keeping them alive via those means ((In cases where something manages to damage them)) unreliable. The damage increases are nice but Bot's low overall damage is still a bit of a factor. Also, personal note, Bots being primarily energy damage is over-rated and heavily countered by their long cooldowns.
Best jack of all trades? Friggin' demons. They aren't really the best at any one thing but they can do everything. Buffs, heals, crowd control, -recharge, primarily non-physical, demons can do it all and do it decently. Only real downside is that constant noise.|
Special mention goes to Beasts. They start off slow but once you get the Dire Wolf and your second upgrade, they skyrocket. The only reason they don't place higher on the damage list ((They're behind Ninja's and around Thugs/Necro)) is because their damage comes in bursts which are dependent on their self buffs. Their survivability is dependent on the same, on buffs with longish cooldowns so they aren't always hard to kill, and when those buffs are down they die kinda easy.
Necro's are an oddity. High damage, decent AoE, decent CC, but some issues with their skeletons knock them down a bit. You can overcome said issues a bit by putting them right on top of enemies so that they are less likely to go ranged first but its not a perfect fix. And holy mother of god, Necro henchmen are squishy.
Last and least, my favorite MM, the Mercs. I love Mercenaries, I really do, but these poor suckers got the short end of the stick. For some reason they decided rocket needed to be on a four minute cooldown, snipe is on a minute cooldown and deals less damage than even moderate Jonin or Skeleton abilities which have far shorter cooldowns, the medic doesn't get the Assault Rifle Upgrade, gets a weaker grenade attack, a long cooldown heal, and a meh mez resistance, and your spec ops love to waste their CC's on targets that are already suffering from CC affects. All of that coupled with lowish damage and almost entirely physical damage. I love them dearly, I do, but boy howdy do they need help.
Finally, /nature doesn't get enough love. -resist, +resist, shields, AoE heals/endurance restore, AoE root passive, long duration +damage and +to-hit, and one of the best AoE heals of any support. The set really needs more credit.