Sailfin mollies. I don't mean those things you get at any old pet store, I have some that are wild forms. They're brackish (impossible to keep alive unless the water is really, really hard and salty). Mixed sex group in brackish community = my homework never gets done cause I'm watching my fish. They are always moving and they seem really happy. And they eat my fingers as well as anything I put in the tank.
Bettas are a close second. Elaboration not necessary.