There were a lot of great games for '06. But the top games were: Gears of War, Guitar Hero 2, Zelda, FF 12, Oblivion, WoW: Buring Crusade. So lets talk about each.
Gears of War was amazing. It is definitly the best game on the 360, and it has smashed all Xbox records (including the records Halo set). The multiplayer is a little lack (only 8 people per game).
Guitar Hero 2 didn't change much except the different things you can do on multiplayer and new songs.
Zelda was also amazing. But when it comes down to it, TP is just a GameCube port with the Wii motions.
FF 12 gave us a new fighing mechanics for that series. I felt it was well done. In the end though, I didn't feel satisfied.
Oblivion is a masterpiece. From the quests to the world itself, almost every aspect of this game is perfect.
And finally, WoW: Buring Crusade. So what do you when you've created the most successful MMO of all time? Change nothing, which is exactly what Blizzard did. They gave us two new races, quests, and now flying mounts, but the gameplay never changed (SW: Galaxies should take note of this). Now with over 8.5 million players, its obvious that it is going to be a very long time before we see the end of WoW.
So in the end, my vote is for Buring Crusade.