Pretty much any and every fighting game.
That's not to say I never enjoy fighting games, but for the most part, I don't have the time/skill (probably) to learn the ins and outs of a character well enough to ever compete with another player. I usually can't even beat your standard arcade mode on any difficulty other than the default 'normal' difficulty.
I respect the simplicity/complexity mechanical mix that a lot of fighters sport, and I respect the people who can master these mechanics, (though I absolutely hate the people who think that this makes them some kind of semi-divine being, and get a huge 'holier-than-thou' hard-on, which, unfortunately, seems to be a large percentage of the people who play fighters online), but for the most part, fighters are not my cup of tea.
I could say the same about Starcraft. I'm fairly decent at most RTS games, but I've seen people play this, and the upper echelon is absolutely nuts. Starcraft is not for me, but I respect the game for creating such a hugely devoted audience.
Also, Dragon Age II. I didn't actually dislike this game very much, and I know people give it a lot of flak, but I respect Bioware for the experiment that was this game. I don't think it was a rushed cash-in to ride the bandwagon of popularity the first game had, I think they were legitimately trying to experiment with the core mechanics of the experience. It's a shame that they didn't just make this some kind of expansion pack, but, still, I give it props. I hope that the third game proves to be a more widely enjoyable experience, though.