Metroid Prime, all the way. The music, the fluidity, the gameplay, the puzzles, the enemies, the brilliantly realized world. Its got it all going for it.
I should say that I have played all the games on the list just within the last 3 years. I was a lapsed gamer until the Wii came out, my last system being SNES. So, I have a "fresh gamer's' perspective on these titles. Probably explains why I don't care about Melee. I did not spend hours playing it with friends, though I have played it a bit with my kids and their friends. I have played much more of Brawl, again with my kids and their friends, but the game doesn't hook me that much. Its too chaotic for my taste.
RE4 is one of the few games that hooked me enough to play it through to completion without a break in between but I wouldn't call it game of the decade, just a really well executed action game.
Wii Sports is a hell of a lot of fun and more innovative than people give it credit for. And, because I'm old and seem to have such limited gaming time these days, I appreciate its pick-up-and-play quality. I'm very tempted to vote for it. But compared to Metroid Prime, it does not get the vote.
Galaxy is great and really trippy, playing upside down and all that. Plus the music is great. Also, I appreciate its pick-up-and-play quality. You can turn it on, fart around with a level or two and get in some quickie gaming fun. But, it lacks the depth and cohesion of Metroid Prime, so again, it does not get the vote. Even with Prime, I can put it in, blast away at a few stages, find a save point and be done for awhile. Its not exactly casual, but at least I can come and go.
Lastly, there's Zelda. Oh Zelda, I so want to love you as a franchise, I really do. And I have tried, oh how I have tried. I have started Twilight Princess, Ocarina of Time, Majora's Mask, Windwaker, the original NES game, Zelda 2, Link to The Past and Phantom Hourglass. I've gotten pretty far in most. I've probably gotten farthest in Link to the Past, but I played it back when it came out. Zelda is one of those franchises I feel obligated to try and love because I can see the high quality written all over it. And yes, its fun and well thought out. I dunno what it is. After awhile, I just get bored with them. Its like there's just too much game or something. Anyway, all that to say that I didn't vote for Majora's Mask.