Witches and hags are my absolute favorite; they're great regardless of how beautiful/monstrous or evil/good they are. If those aren't proper "monsters", I also like any variation of house spirits (i.e. brownies/kobolds/zashiki warashi/domovoi etc.)
Gumiho or Nine-tailed Fox. Granted I only fell in love with them because I liked Kurama from Naruto, but the story behind it is really interesting and I love all the depictions of it