My Marigold Platy!
Hands down the smartest 'social' fish in my tank.
She learned to 'spar' or show aggression from my gourami's (line up parallel to opposing fish and slap tail at it) and uses this teqnique against other fish now to show dominance.
And she finds bottom food faster than any of my other fish. She even holds her own against a much more aggressive Apistogramma Agassiz thats a bit bigger than her. Despite the aggressive fish, she holds her ground and gets that food!
And believe it or not, She Schools with my Praecox (dwarf) rainbows a lot of the time!
I wouldn't have thought a mere platy to be so smart/socially adept... but my one and only in my tank certainly proved me wrong!