the three resist check monsters:
Ice Clan (a fan favorite)
Corruption Mage (bonus points to this guy for running away any time he takes damage, generally just to the other side of a huge wave of other enemies so he can go back to filling the screen with poison)
Pest (these are the guys in act two that throw exploding spikes everywhere when they die)
Do you have your resistances maxed? No? Then these assholes are going to fill the screen with their element and instagib you. On the bright side that's only 3 of the 4 elements; if something did obscene lightning damage it was far enough into the game that I had already maxed my resists anyway
honorable mentions to those act 3 enemies I can't remember the name of. They make this big wall in front of you before they even get on screen. It doesn't do any damage, it's just annoying having to randomly wait 5 seconds for the wall to fall