Any Sonic Heroes special stage, because stupid random shit happens like you just spin up the tube and stop dead in your tracks, jumping slows you down like nobody's business, the bomb rows in later stages have extremely narrow gaps and also stop you dead in your tracks, and your characters have the acceleration characteristics of a forklift. You either get good luck with boost balloons and catch the emerald near the start, making it way too easy, or you get left in the dust and have to pray you can catch up before you run out of time.
Sonic 2 special stages are a bit unfair because they depend on memorization, which means your odds of clearing most of them on the first attempt are not good, but once you know them they're totally predictable. Plus the game gives you all sorts of attempts at them since any star posts can get you back to them if you can scrounge up 50 rings. I like the Sonic 3 special stages the best.