Not sure I'd call it "guilty" pleasure since the genre's so popular but uh....
I absolutely adore Monster Girls and Monster Guys.... The guilty part would probably be I like the more monstrous types over the kemonimimi style.
I also absolutely freaking hate it but I can't stop myself from getting a cheap thrill out of cookie cutter Supernatural Romance stories. That's probably the guilty pleasure.
Oh, and harem stories. Especially reverse harem....