There are so many fantastic games, it's impossible for me to determine exactly which one is best. Some of the top candidates include Bioshock, Civilization IV, the first Splinter Cell, Halo: CE, Gears of War and Football Manager 2010. I also think Fallout 3 and TESIV: Oblivion are great games, although I'm usually not a big fan of RPG's.
Battlefield 2 - it has unlimited possibilities to fuck up and kill your teammates!
Seriously, I don't think I've ever had this much fun with any other game rofl.
Other than Rayman... ohh that's a toughie... I'd say for me it would be Worms world party, Worms open warfare 2, Jak 2, the Simpson's hit and run, The Sims 2, Midnight club 3, Smuggler's run 2 (this is just from the top of my head)...
Best game I ever played is definitively Psychonauts, and then I'm not even talking "apart from Rayman". Other games that are definitively on my top list include Beyond Good & Evil, Brütal Legend and Munch's Oddysee (and also Rayman durr.)
Gosh there is so much good stuff on the indie marked I'd like to try though. I wish I was able to pay for and play the full versions of stuff like World of Goo and Machinarium. I'd also like to try Devil's Tuning Fork, which is free, but I don't have anything that runs Windows right now. :<
I share the views of several of the above posters, but one game that hasn't been mentioned yet and definitely deserves to be is Shadow of the Colossus.