I think damsels are WAY underrated! Often the entire group is judged upon the merrits of the more well-known, and yes, aggressive dominos & yellowtail. There are SO many species of damsels out there. My orange tail sunshine damsel is the meekest fish in the tank, and always getting beaten up on by the others, but takes it all in stride and with bounces back for more.
I think other fish that are underrated include the hogfish group... of which I am a HUGE fan of, since they remind me of a wrasse's personality. I like goby's & dragonets as well, but I think there are so many people who have them, not much needs to be said. It all depends what you mean by "underrated".