All Triggers, all Clownfish, all Tangs, most Angels, ect. are open water fish.
If you are willing to devote an entire tank to one fish, get a beautiful Undulate Trigger. They are very intelligent, but unfortunately they are rediculously aggressive. Watch out for "love bites"; Triggerfish can draw blood from a human easier than fire can burn paper.
Humuhumu Triggers (both rectangulus and aculeatus) make excellent "show" species, as do the other Rhinecanthus species.
Xanthichthys Triggers are very colourful and peaceful, and Melichthys wouldn't hurt a fly; unfortunately, Melichthys grow very large and a 77 gallon tank isn't enough in the long run.
Odonus niger is another beautiful and fairly docile species, but a tank of at least 6' is needed for this giant.
-Lynden