One of the worst for me was when I was riding Top Thrill Dragster in the rain; it hurt so bad (and it was only a drizzle too but going 120 mph it can still have a big impact).
Ever since winter 2003 when Cedar Fair bought Six Flags (which came to be Geauga Lake then came to be a GHOST TOWN) my appreciation for Cedar Fair vanished completely.
This is a hard question even for me; there are just so many excellent rides at Walt Disney World. Tower of Terror, Haunted Mansion, Space Mountain, the classic Splash Mountain, Rock'n'Roller Coaster, and so on. I can't choose JUST one.
I say tear down Mantis at Cedar Point and put something that is actually a good ride. That coaster is just too rough; too much head-banging and too much pressure on your legs to the point that you can't even walk straight after getting off the ride.
I remember when Eiffel 65's album came out in like 1999 and I was freaking out because I was young at the time and I thought they were great. Good ol' techno haha.