Metroid Prime, leaping from 2-D to 3-D and still remaining just as incredible as it always was.
I'll admit I wasn't completely sure about the idea of a 3-D FPS/Adventure but Retro made believers out of me with this game.
The graphics were the first thing I noticed, it's safe to say they are up there as some of the most beautiful graphics to grace this generation of gaming. Every individual had its own feel to it, which made it easier to get into the game, whether your platform jumping hoping not to miss and fall into lava in Magmoor Caverns or Grappling Hooking across some Phazon in the Phazon Mines, every area is incredible. The enemies are well designed and oh boy, the boss fights... incredible.
The music and sound in general is another strong point of this game, it brings you into the area with fitting tunes. Phendrana Drifts has possible the best music though, that's my opinion. But all the music is great, the boss battle music pounds away as you side-hop firing your Charge Beam at the boss, just very well done sound.
Gameplay is great and with the perfect blend of puzzles, boss fights, item-collecting and backtracking its hard to imagine why this game wouldn't be entertaining. And with the added bonus of finishing it once then tackling Hard Mode adds to replayibility, and trust me, the difference is noticeable at parts between Normal and Hard. And the hunts for all the items can keep you playing it, attempting to get 250 missiles is interesting and sometimes a long process, depending how good you are with this game.
Overall this is a must-have title for the Gamecube and if you do not have it then I recommend getting it, it's Player's Choice so you'll get it cheap, well worth the money.
So basically... buy this game if you haven't already.