I'm not really a fan of her (as in, I don't make a point to listen to her music, I don't dislike her in any way), but I do like Firework a lot.
Dark Horse actually really annoys me, though. The only thing I sort of like in the song is her voice. Everything else is "meh". I like some of the lyrics, and some of the visuals in the video. Otherwise, though, I really don't see the appeal. It's not fun to dance to, it's not unique, it's not especially moving one way or another. It's just kind of there and I find it mildly annoying.