I already voted for Halo, but I guess I'll defend (or at least shed some light) on why I voted that way.
I will say Halo is a good game and, arguably, the best MP console FPS, but at that time, being the best console FPS was like saying you won a race vs. 1 other person. Sure, you won, but the other ones just might have sucked.
What Halo brought to the genre were a few gameplay features and the fact that you can play 4 players without an internet connection or needing to lug a computer some place.
The gameplay features are minimal and boils down to preference if you like them or not. Regenerating shields, grenades and limiting yourself to two weapons. Not my cup of tea (these features make the battles take a while longer and thus, less exciting to me) but for some it might be.
The main reason why I think Halo is over rated is because, while the ability to play 4 players pretty easily in the same room is awesome, that feature has been available for FPSs that already existed on the market that were close to a decade old at that point.
So, Halo was a "game changer" in the fact that it was on a console and not on the PC. I think any game that goes from "HOLY CRAP IT IS AWESOME!" to "meh" just from simply changing platforms (when the platform change does nothing to alter gameplay) is overrated.