I'll vote Uaru as well. Mine is by far the most social fish I have. Plus you could also keep some other smaller friends (cleanup crew) in there with them without a problem.
At first I was thinking a 75 might not be enough for an Uaru but they're not super active fish either so now that I think of it it'd be fine.
Really though, there are lots of social fish that would work just fine on their own, so get whatever you think looks the coolest. That's personal preference, not something any of us can give advice on.