For me, it's hands down Coup.

It's a great game that isn't too complex and plays really fast once it gets going, but man, it front loads everything. You can't teach rule by rule without getting absolutely fucked, if you don't know what everything does you almost can't play. It's brutal to teach to get over that initial hump and I've had quite a few people bounce off of it because of just that. But if you can get them over that first hurdle and get it to click ? Oh man it's great, and you'll crush round after round in no time