As far as design goes, probably Thoraxians. Yeah, I know they're just your bog-standard, generic, hive-minded insect race 101. But the mood swings of the queens just make them FUN. In the Dwarf Fortress way.
As far as the race I like the most it's without doubt Skylaxians. I've always been a science geek when I play 4x and pump way more resources into science than I should and the Skylaxians are smart AND friendly. Can't go wrong with that.