One of Rare's greatest achievements. It had an incredible atmosphere, memorable music, a great story, and pitch-perfect gameplay focused on exploration. I even loved the much-maligned multiplayer aspect!
One thing that's great about it is how well it balances tons of different elements. It's got shooting, platforming, and free-roaming, with several minigames that could be games in themselves. Basically, it's like every good idea on the N64 crammed into one game. I still don't know how they did it.
It did have the same flaw I'd come to expect from Rare: completing everything got to be tedious, and fighting the final boss pretty much required you to 100% the game. This was a bad design decision, and one that prevented me from finishing the game. I loved the time I spent with it, though, and it will always go down as one of the best N64 games to me.