How to answer this question...

I love killing the skeletons in cathedral of bones, and raising some casters as specters is awesome too.

The monkeys are really fun too, because I can round up a couple of packs and AoE them all at once.

EDIT: Like some people have already pointed out, the sandspitters are really annoying. The ones that are crabs first and then turn into sandspitters are almost as annoying, atleast the sandspitters that come from those don't have as much hp/armour.