I play a lot of MMs, so to touch on a few things that were missed:
Necro - Lich, the T3 does not have a lot of damage, but does perform a ton of crowd control and debuffing which all helps survivability. I am running an incarnate Necro/Time right now, and it handles the job very well. Slotting the upgrades for heal really made a huge difference with the life drain attacks.
Demons - they are very tough, in my mind, the tankiest individual pet without adding buffs. The T3, the Prince, adds in a fair amount of crowd control. Noisy. Really noisy.
Thugs - Keeping the arsonist alive is your job. I like Spirit ward for that. The T2 has a defensive buff that affects the minions, but not you.
Ninjas and Beasts - Quiet, thank goodness. If I could keep them near me, they would be fantastic, but they chase. My best advice is to keep them attacking a specific target.
Bots - I like Bots a lot, but in the beginning, they feel slow. Noisy. They clank every step they take. There is a huge wait between attacks that I never noticed before. The T3 is great for AOE damage, and seeker drones to take some hits. The T2s bubble your team including you, adding to survivability and then heals.
Mercs - Noisy guns. If you like that, cool. I can't decide whether I love or hate mercs. New Mercs, not old. Initially, they fire at a massive rate, making other pets types feel slow. For the first couple of levels, they are definitely my favorite. After that, they lose a lot of luster. No heal til 18, no resist, I don't know numbers, but the damage doesn't feel spectacular.
Frankly, I'd rather have a pet group that can semi-self support than one that does super damage. I like self buffers. I like sets with a healer. You will get mezzed, stunned, held. You will have times where your heal is down. Replacing a minion isn't game over, but its irritating and having to replace multiple minions per fight turns me off of a set fast. Out of all of them, Necro and Bots are my gotos. Tough and heal themselves. Lich just adds in more that I want. Enjoy the game. Have fun.