Growing up, I was generally one of the best or the best out of all my friends at the games we played, be it sports games, FPSs, fighters, etc. I think that it was mostly just talent / good reaction times / good awareness of where my dude was / whatever you want to call it. I've definitely started gaming less as I get older since I just don't have the time anymore, but I'm still pretty good (probably the best out of my friends, but not great by any means) at MOBA games like DotA, HoN, and LoL, which are the games I play the most of now.
Notable games that I was really good at:
Halo 1 & 2 multiplayer - back when we would all get together, hook up 4 XBOXs, and LAN Halo, I reined supreme. After 2, we didn't play much anymore, and I never played the multiplayer enough on the other ones to really know the maps.
Super Smash Bros. (original) - I played the crap out of this game, and could beat anyone that I knew pretty easily. I didn't own a Gamecube or Wii, so I haven't played Brawl or Melee very much, so I can't really say how good I was at those.
Tekken Tag Tournament - I can't even count the number of quarters I spent in the arcade playing that game. In my home town, I participated in the tournament held in that game and placed second. I was fairly proficient and could usually beat the game with any combination of two characters, but my main duo was King and Lei Wulong. Yeah, I was that dick that would catch you in a throw combo and just never never never let you go!