I cannot say anything about the Assassin. I only tried Rogue, Sorceror and Necromancer. The "safest" to play with is the Necromancer. As you've said, though, in Torment he's quite slow : Skeletons are good tankers, but the monsters have tons of HP, especially Behemoth ones (I remember, for example, my Skeletons missing 9/10 hits on Behemothed Blackjade, and that hit that was successful only took her 6 hp...). The gameplay with Necromancer could be faster and less boring in Torment if the Revive system would get some improvements, because they can deal damage even to Behemoth bosses (in my opinion, skeletons should be weakened while revived should be permanent, followers, but lesser in number, i also think that the Necromancer should get a cooldown time on Corpse Explosion to balance the situation : you pass from moments that your char is a demigod to moments you wish to delete it XD)
As for Rogue, i didn't manage to pass normal difficulty : i probably built her badly and with crap stuffs, and monsters just needed 1 hit to take me 1/2 of my hp pool. I'm defenitely NOT the one to ask for rogue
Concerning the Sorceror, it's quite fast and strong, with a very useful (and curable, that's very important) golem. It's downside is that he deals no physical damages (the Necromancer, through its skeletons, can) and he will have trouble in the late game, since it's full of tri - tetra immunes.
These are my experiences, though. There might be people who could say the opposite
