Competitive Shooters (Especially Sniping) and, unfortunately, fighting games.
With Shooters, it's an issue of not having very stable aiming. Not as big a deal with PVE, but when it comes to PVP with that added layer of human enemies, I just can't.
For fighters, it's not so much that I'm bad at them, as I understand the fundamentals and can perform at a decent level with that, even against people a bit better than me. However, the "Bread and Butter" style seems to allude me in any fighting game I play, as I play too much on instinct and end up button-mashing a bit. I generally have an idea of what I'm aiming for, but execution often alludes me. This same concept translates a bit towards high-level Character Action play as well, but I don't care as much about that since it's PVE anyway.