My most favored opponent is Bretonians. There not too nutty in any category. Most of the players, in my area at least, like the whole honor thing so I rarely face dirty tricks. They have a variety and flavor that is always fun and finally I love the look. To be fair it may also be the nostalgia of fighting them. I started in 5th when the starting box had Lizards and Knights.
I like playing against Orks and Goblins because it’s always an interesting game and you rarely face the same army twice. Of course with more than four times as many lord choices and twice as many core as us that isn’t as hard as it seems.
I enjoy fights against Skaven, Empire and Dwarves right up to the point when the pull out the Lets kill Lizardmen with the dirtiest tricks possible lists.
The rest are hit or miss mostly depending on the opponent.