I found a combination of all three was handy: Titans for bubbles, to protect us when all hell breaks loose (saint 14 is super handy for this). Warlocks using nova bomb can clear mobs easily (striker Titans can also do this). Sun singers I found were handy but mostly as a grenade spammer than for their revive (not so good when being attacked by constant mobs). Hunters with their invisibility as OP says is great as medics, but also bladedancer helps with the weaker enemies that run in mobs. There is no best class but I feel that the biggest challenge would be if you ran PoE with three of the same class!