GCube ESSENTIAL
This game has TONS of unlockables...too many if you ask me. You unlock them by winning arcade scenarios and challenges. It can prove to be tough since on some large maps the tougher AI can prey on the weaker AI to score more kills.
If that turns you off theres still the custom arcade and the mapmaker. Now I heard the mapmaker is alot of fun, but I really don't use it. But the Custom Arcade is great fun, think of Perfect Dark 64 renovated by Bungie.