If not something video game related, it would probably be...
Actually...I don't really know. I'm physically fit and a great swimmer, but I'm not a powerhouse or the fastest on land or sea. I'm smart, but I'm not quite a genius (and I don't apply myself very much). I'm socially okay but not fantastic...
I'm above-average in nearly every aspect, even way above average in intelligence, but I don't truly excel at any one thing. I guess I'd just use a jack or something, a Jack of All Trades. Maybe something leadership-based...?
Edit: After looking at some of the other posts, I would say that my cutie mark would probably be something related to logic and reasoning. I'm analytical and like to make sure that all of my arguments are sound.