5. Metroid Prime (GC)
Metroid Prime is so good for a variety of reasons. It's not just the basic idea of the game or anything as simple as that, it's all about the sum of its parts. It wouldn't be so good without the same environment to slowly explore, or with some different enemies, or different weapons, or any other individual part that makes the game so great. If one thing helps the most though, it's the sense of isolation that the game gives you. You are alone as you explore this hostile environment, and you sure do know it. That's something that not many games can accomplish, but it was done to perfection here.
Clue for number 4. Take that, Jaws.