Aliens for me. Like someone else already mentioned, the hungry vampire thing gets a little old. Also the fact that I have to give them weaknesses. I wish there was an unmodded way to get around that as sometimes I'd just want an overpowered vampire.
Another thing I don't like about vampires is the fact that their genetics only matter as long as their offspring is a vampire. A human sim with a vampire parent can't get their glowing eyes for example (nor teeth, ears or markings).
If that could be passed down that would make them more interesting to me, especially the eyes. Like even if you create a vampire in cas, that has glowing vampire eyes in both forms, their eyes change to a "human" eyecolour when turned, boring.
I had a blast in ts3 mixing different supernatural because even though we didn't have hybrids then either they could at least turn out very cool looking and you could tell if a sim had vampire, genie and fairy genetics etc for example. Loved that.