Probably every map that was created by Darth Human But there are lots of people that know how to forge a decent minigame, actually, including people like Sam The Great and Prodigious 7k. I'm actually planning to produce some more minigames myself, next Summer, when I'll be free from school and have much more time to work on maps.
Must resist urge to ADVERTISE Grifball is an OK game except that the randomness of the hammer clashes sucks all the fun out of it. Also, people who insist on abusing the fixed spawns. Let's just say Grifball has a lot of problems. Crash-up derby is a reliable standby of a minigame. I really like it as a breather between serious games, though I don't really understand lobbies that sit and play it all night... IMHO, the biggest lapse in minigame-making has been that not enough people have done fun things with the basic Haloball setup. Hockey isn't fun at all, and it seems to have perhaps dissuaded people from trying to make their own variations on the basic "put a ball in a goal to score" theme. I mean, I'm all for creativity, but I think somebody could make a truly enjoyable ball-based game, yet few people are trying for whatever reason. A lot of the minigames I see and that get hype are more creative, but perhaps not as mindlessly entertaining (well, depending which game of course).
I still have yet to play jump rope. It looks incredibly fun though. Over, throw me an invite some time you're in a group that's playing it.