I picked up Halo Reach at midnight on the day of release. By 8:00am I had finished the game on Legendary. On multiplayer I've always been pretty good, but put a sniper in my hands in any game (except for Black Ops, because the snipers on those are so freaking weird that I can't get the hang of them) and I'll wreck shit with them. I'm good at getting the first shot off in firefights, but I've always been an accuracy kind of guy instead of a spray'n'pray, which means I'll take the time to line up a shot and hit every time. I've got a 45% accuracy rating in MW2, and my rating in Reach is similar (with snipers it's closer to 80-90%). The issue with that is in close range battles I'll always hit the first shot, but instead of just firing everything I have I'll take the time to line up the next shot again, and that only works about 50% of the time. That said, I always come out with a positive k/d ratio.
As for other types of games, I've beaten all of the BioWare games I own on the hardest difficulties, I started out gaming on RTS's and have gotten damned good at them (although that's mostly against the AI, since I take the time to build my force/base logically instead of building for speed and killability; I'd say I'm decent against players, but not the best by any means. That said, get me a good teammate who can distract them until I get a big enough force up and I'll be the one to finish a match every time). You can't really judge how good a player is on games like Oblivion, but I like to consider myself pretty good at those too.
The games that I've never been very good at are sports and racing games. I can kinda hold my own at racing ones, but hold my own is a very loose term. And I don't understand sports games at all; I'm much better at actually playing most sports than playing games of them.