Anything online-only or multiplayer-only, really. I play games to get away from everything else, not to socialize. Competitive multiplayer especially isn't fun because I don't have room to play at my own pace, and one of my favorite things to do is figure out how to win things with weaker characters, classes, et cetera, and I can't really do that if I'm playing against people who are using the best builds in the meta all the time.
I'm also not a fan of point-and-click games, visual novels, rhythm games, survival games, horror games, or anything with permanent death.
Point-and-click games and visual novels don't have enough gameplay, which to me defeats the entire point of a video game.
Most rhythm games focus on music that I'm not a fan of, but there have been a couple like Audiosurf that let me pick things from my own music.
I don't like focusing on inventory management in games in general, so I don't like most survival-craft games like Minecraft. I also don't like managing hunger or other needs systems (e.g. thirst or oxygen) since they constantly tick down far faster than it would take in real life, and I tend to play games slowly. It makes me a little sad because I love how these games do world generation, but everything about the actual gameplay isn't interesting to me. I have played some hybrid-genre games in this genre that I enjoyed, though, like Terraria and Starbound.
I don't find horror games that scary, but I don't find them fun, either. I don't like feeling powerless in games and like to be able to actually fight and take down enemies instead of running and hiding from them. I also have basically no reaction to jumpscares, so that element of the genre isn't interesting to me either.
Finally, I don't like permadeath as a mechanic since it pushes me away from experimenting with new builds and stuff, something I enjoy, in favor of always playing it safe. I'll tolerate it sometimes if the game is easy enough and runs are short enough, since a lot of games with permadeath are roguelikes that use procedural generation (a mechanic I love) in really interesting ways, but I'll always turn it off if given the opportunity.