Any fighting game. I'm horrible at games like Mortal Kombat.

Simulators, doesn't matter what type of game it is. A simulator like ARMA will seriously kick you're ass the first 15 hours you play the game, and even if you good, you will still end up getting you're ass kicked.  No aim assist and high adrenaline can cost you the victory.  Even racing simulators can kick you're ass if you aren't experienced with it. i have no final answer other then simulators, and i love them because they challenge me

2D platformers, believe it or not.

I like a lot of them, finished most I played and the original Yoshi's Island is in my top 10 favorite games of all time, but I'm really bad at them. Even on NSMB, which is supposedly easy, I die like a billion times, let alone a hard one like Donkey Kong. 3D platformers feel much more natural to me and thus am way better at them.

I'm rusty at some other genres, but that's mostly because I don't play them very often anymore. These are FPS's, rhythm games and fighting games mostly, but I used to be skilled enough at Call of Duty, was quite decent on SoulCalibur and was excellent at Guitar Hero so I know I'm perfectly fine at those when I put some effort in it.

Bad : Shooters and RTS
So-so : Action-adventure, beat 'em up and rhythm/music
Ok : Platform, fighters and TBS
Good : RPGs, driving, sports and puzzles

In general, I'm better at slower paced games.

I'm terrible at traditional strategy games, like Civilization, Age of Empires, Starcraft and such.

I get the gist of RTS but I'm not great.

Sports games mainly, don't play them or have any interest in them. When I play my friend (if I play) I usually stick to a few buttons and hope I don't lose by too many. :P

Fighting games and shooters. I'm great in single player but I usually get destroyed online.

shmashups and fighting games

the speed was a problem in both games,i am a slow gamer

RPG's

FPS and TPS, shooters is a genre I just can't get into it.