Twitchy/tactical shooters, like CoD or CS. I'm terrible at them.
It's not a matter of having poor reflexes, map knowledge, or weapon selection; I think it's a matter of psychology for me. I enter into a tactical shooter with the intent to play tactically: systematically checking corners, clearing areas, setting traps, playing defensively, or working as a part of a team. Then I get wasted by the bunny hopper/dropshotter who runs into the room blindly and either is spraying and praying or physically incapable of anything but headshots. Or, I end up in a really good position across the map from where the action is; conversely, I'll be roaming around the "smart" areas to do so and everyone else is engaging in gratuitous slaughter in the middle of the map out in the open.
I just can't wrap my head around it; it's disconcerting, annoying, and quite frankly often enough a complete immersion-breaker. That's in very stark contrast to more action-based shooters like UT, Quake, or Halo in which I'm actually pretty good.