There are four writers in gaming who I consider to be really great, and Tim Schafer is one of them. Psychonauts, sad to say, remains the only Schafer game which I've played all the way through - my Windows registration threw up on itself not long ago and I never picked up Brutal Legend. But that's really not the point.
Psychonauts is clever, pretty, witty, rude, and deeply introspective. As much as it is a fun game full of fun environments and fun scenarios (Waterloo alone), it is also a thoughtful meditation on the mind in general and mental illness in particular. It's uproariously funny in places and downright terrifying in others, full of characters and concepts you will unquestionably care about because you cannot help but care about the subjects that they are dealing with. There is a particular style to the humor of Psychonauts, which may make you question exactly what it is that you've heard, and it's one of the most endlessly quotable pieces of media outside of Monty Python. Great stuff from beginning to end.
Now, the game's primary mechanics are solid, but near the end they often leave a bit to desire. Why, then, does the game place so high on my list? I'm a sucker for solid writing and Psychonauts has it in spades. Funny and sincere and to-the-point, Psychonauts is unique among games in that it does not make me reconsider how I look at games or the world around me, it made me reconsider how I look at myself. That's something that no other game can lay claim to.