Anything and nothing.
By default I don't class myself as ridiculously good at anything. There are doubtlessly people out there that could kick my ass, and I doubt I'm good enough to enter into professional tournaments, so I'm not ridiculously good at anything.
However, there are times when I will enter 'The Zone' where I do become ridiculously good at whatever I'm playing at that time. I am actually able to focus, as opposed to normally where my mind is constantly wondering no matter what I do - so I do pretty much everything on autopilot, and my reflexes and endurance improve. If I'm playing Guitar Hero I'll not miss a note for as long as I'm in said zone, in Starcraft I come up with better tactics than normal and am able to effectively Micro and Macro manage at pretty much the same time, as opposed to being fixated on one or the other. In FPS I get the first shot off, and dodge half the other shots, and in space fighting sims everything just flows so that I don't get hit, perform crazy maneuvers and make my opponents crash whilst staying 100% fine myself.
Sadly, 'The Zone' is a very short term and erratic thing thanks to how my mind works. It could appear at any time, and can last anywhere from 30 seconds to 30 minutes, so generally I'll get either 1 good fight, or one good round, but then I'll go back to being just plain average, or below average - dependent on the game.