I'd say probably a triggerfish, but I did have a very interactive oscar as well.
Generally speaking this question favors reef fishes. They are, for the most part, more behaviorally-intelligent* than most freshwater fishes. Cichlids come close, but when compared to things like snappers, groupers, triggers, puffers, etc...it makes them look like they ride the short bus.
*I'm defining this by how much of their brain power is concentrated in the telencephalon and/or cerebellum. That's not to say that other fishes' brains are bad, they're just wired differently. Some mormyrids have a greater brain/body-weight ratio than humans! But the majority is devoted to electroreception and production of electric fields. The human "telencephalon" (referred to as the cerebrum) is comparatively huge, and that's one of the reasons why we're smart and awesome.