Psychonauts ranks on my personal top three as one of the best (if not THE best) video game I've ever played. It's so incredibly imaginative, brilliantly written, hilarious, exciting and event quite moving at times.
Even though it received decent reviews at the time it was released I still think it was a severely under estimated. This is not just a "fun" game - it an absolute masterpiece where everything just seems to come perfectly together. The characters are amazing, the level design is genius and the visual style of the game is absolutely beautiful.
If anyone asks me what I like about video games, Psychonauts is the best answer I can give.