1. Zululand. Their UU may give a strange GA time, but it allows them to build a lot of them, and with 2 movement, those Impis are in your territory fast. High aggression level means war.
2. Rome/Greece. I like Conquest victory and it's harder to win when they have good defending units early.
I would have added the Germans because they always declare war on me very early but since I find them easy to destroy, I appreciate it.