I'm not convinced that I'd call these guilty pleasures (and I don't particularly care for the notion of guilty pleasures anyway), but Rocky Horror and Taboo.
I don't think there's a correlation between something being considered a guilty pleasure and how well-known it is. Mamma Mia! is probably one of the most widely recognized musicals on Broadway today, but I'd definitely but it in the guilty-pleasure category (I personally happen to despise it, but nevertheless...). And, again, I really don't think anything should be considered a "guilty" pleasure; what on earth is there to feel guilty about? :)