Any shooter game, be it first or third person. I just can't get used to the combination of a steady hand to get a good aim and the twitch required to react when someone fires at you. Plus, because I hardly ever play them, I just don't know the maps as well as the people I'm playing against in multiplayer. Doesn't help that I just suck at them in general and can't even get past the third stage of any in single player without co-op help from friends. Yeah, I'm that guy who holds the others back in co-op when it comes to multiplayer.
I also tend to suck at fighting games, but only in multiplayer. I do alright in single player, and I like to play fighting games, so I do some practice to get better (I believe I even have a winning record in SC4 between Xbox Live and local matches), but I didn't really grow up with fighting games such as SF2, so it's a long learning process.