I know I will get a resounding chorus of "You're doing it wrong," but threesomes. I've only had one (or two, if you count the one time my friend bailed on the girl he was with and she decided to join me and my girl, which neither of us really wanted)
It started out promising, but I think the gals were just not really in to each other. hey didn't do anything more than kiss, and it ended up with just me doing one, and then doing the other. Which I am not complaining about. But individually neither of them was extraordinary, such that taken alone, they would both be considered very underwhelming. So taken together, it was rather disappointing.
Still working on a follow-up with better talent, but being married is hampering that goal.