Let me start first.

Me = Monk, Act 4 is the hardest. Reason is because of his -75% healing effect and monk had to facetank him. Might be easier for Throwing barbarian and range characters.
Sokahr might give quite a problem to range, and of course the solution is to shoot him real far.
Odeg's jailer+molton will give problem to those without the ohshit button.
Xah'Rith is no longer a great threat after the illusionist remove from the latest patch.

That's my though.