Metroid Prime 2: Echoes (Game Cube, 2004)

As much as I love this game, I must first give you a warning: If you didn't like the first Metroid Prime, you'll hate this game.
Seriously.
Having said that, it's really a shame to hate Prime I and II, since they're some of the best Gamecube games of all time, and some of the best games ever.
Metroid Prime II follows the continuing adventures of Samus Aran as she lands yet again on a alien planet. One thing leads to another, and she is forced into a war between a race of moth-like guardians of the planet, and inter-dimentional darklings, called the Ing.
The single player is still very meaty, with at least 20 hours in it, without all the pickup and upgrade hunting. But, with Prime II, something very different has been introduced into the Metroid universe.
Multiplayer.
For those of you who have played Prime, you know how awkward this sounds. And for those of you who don't, read on.
Combat in Prime I and II is done by auto-targeting you enemy, and blasting away. Although you can get away by quickly jumping out of the way, or turning into a morphball, the whole thing is very awkward. Also, with only a handful of average maps, only two modes, and no AI support, the whole thing comes off as an interesting offshoot of Prime, but nothing great.
Overall, this is an incredible game. Do yourself a favor, and buy it. And if you don't have a Gamecube, buy one and play Prime I and II on it.
I certainly didn't regret it.