Am I only the one here who does not have problems with damsels? Should not say that- famous last words! lol. I never seen them attack anybody, very peaceful little fishes (blue tailed damsel). Maybe because they just juvenile, and will turn evil eventually?
I don't have exactly "least favorite fish" yet. However my royal glamma most aggressive: picking on black molly all the time (converted from FW, when I cycled tank) and recently ripped his lip.
Watchman goby: fish with lots of character but you rarely see him during the day. You wake up in the morning and just find loads of holes in mountains on the sand bed he diged through the night. A bit of a nuisance. ;-)