The first Halo was definitely a step forward for the gaming industry as well as for us gamers. It is one of my favorite games, and probably my favorite campaign of the series. The multi-player was still fun as all hell, but it was almost too simple. Who could ever forget hearing that badass menu music for the first time?..
The other games on here I have also enjoyed, God of War was good, but single minded, lol. They are all great, but they all have their flaws.
Honestly, if I would have voted, I'd have voted DOOM. IMO, if it weren't for DOOM and Wolfenstien... who knows where us first person shooter nutcases would be.