We used to play this joke on all the kids in my fraternity turning 21... if someone offers you a tidal wave say no. The guy buying gets a huge round, like 8 shots. Only 1 is vodka, the rest are water. The birthday boy gets the vodka, everyone else gets the water, so when you clink and go bottoms up, the other 7 guys throw the water all over you and shout "TIDAL WAVE!" while you're choking on the vodka. This sounds stupid but at 1 AM its the funniest thing you've ever seen.
Also avoid - three wise men (or any variation), cement mixer, prairy fire, and of course the blow job shot usually purchased by your smart alec female friend.
Adam