Perhaps Rap? This is actually a pretty difficult question for me.
Theoretically/conceptually, my least favorite genre is country. But there's a lot of good country musicians that make decent music.
Realistically, my least favorite is rap. Mainly because of the tons of awful artists. That, and rap is really more of a lyrical style than a music genre, meaning it can be put on top of anything, but most commonly you get urban hip-hop type stuff, which doesn't interest me at all. But mix it with another genre? That shit is gold.
Two songs that stick out:
Libera Me from Hell - This song has a pretty unique thing going for it, that I haven't really heard anywhere else. It's a beautiful blend of genres that sounds good every time I listen to it. It helps that it plays during particularly epic parts of an epic anime.
Eminem - Rap God - Eminem tends to be a hit/miss for me. Some of his stuff is really good, and then the other stuff is garbage. Rap God is one example of something that I find fantastic. It still falls prey to the "bland background", but the rapping itself keeps the song interesting all the way through. Something that even non-rap songs have a hard time doing for me.
And as always, I like to describe the stuff I like as "interesting". It's not the best word to use, and some people don't quite understand what I mean by it. But there seems to be some sort of underlying theme that makes everything I like "click". It makes finding stuff I like difficult, but efficient.
Edit: After reading/watching the responses, I'm not sure what I expected. Hubski, you always disappoint when it comes to music. I feel like I'm a black sheep here. Damn music anhedonia and obscure interests.