For me, it's definitely the sports genre, and to a slightly lesser extent the first-person shooter genre.
I'm not a fan of sports to begin with, so the sports genre has almost no appeal to me whatsoever. Plus, the genre just feels quite recycled and is very in-the-box for the most part. Basing a video game off of a real-life sport doesn't exactly leave much room for creativity considering you have to adhere to the rules of the sport.
The FPS genre, on the other hand, has become quite stale in recent years. It was once a genre that I somewhat liked, but now that FPS games are mass-produced thanks in part to the Call of Duty franchise, I have trouble telling one FPS from another. There are very few FPS games that I actually like, and those that I do are more outside the CoD box: the Metroid Prime games (technically not true FPS, but very well-made), Goldeneye (N64), and Halo to some extent (I've only played the multiplayer, but I enjoyed it compared to CoD). That's not a lot of games.