Halo: The game that prompted me to purchase an Xbox to start with. Wish more folks would design games like this.  It certainly threw everything I thought I knew about game design out the window.  No increased abilities as you progress, limited puzzles, no real bosses, no jumping puzzles, limited weapon carrying ability, coupled with great graphics, game play and one hell of a story.  Who knew.  This is a must have.

Halo 2: The single player was almost as good as the original and the multiplayer, especially on Xbox Live make this a must-have.  Only wish co-op was available in System-Link and Xbox Live.  Playing on 360 in 720p on an HDTV is stunning. This is a must have. 

Halo 3:  Eagerly awaiting this game.  Bungi has released a cool announcement video (as well as a making of the video video).  Currently set for release in 2007 - probably late 2007 (holidays?).