Most Fun: Most every toon I made was hella fun at some point or another, sometimes just based on if I played with a good team that night or not, but if I had to choose, I think I'd go with my Bots/Dark MM, "Watership Doom" who rocked a creepy bunny/Doctor Doom look. I have always been a fan of the fun an MM gets from standing back and watching their pets wreak havoc and destruction, and Bots were really good at that, while Dark just exacerbated the problem.
Most Powerful: I was never much of a builder until near the end so I never had many toons that I could say were powerful in any form of "power gamer" sort of way. One of those few though was my namesake stalker, Energy Melee/Super Reflexes. After finally yielding to the thought that building builds might build the fun, I sat down with him and after a little work and careful choices, I came up with a stalker who could tank for a team fairly well, something I sadly didn't get to do more than a handful of times - by the end, population on red-side, at night, was pretty much reserved for the "play Incarnate or go home" crowd, when there even was a crowd (I say crowd... a "handful" more like...)(actually that was mostly the blue-side crowd as well). Sure, I might have moved him to blue-side, but that would have felt like an injustice.