To put this simply, everyone who is arguing against this obviously has not spent very much time with Halo 3. Even spending just a little bit of time in Forge, the theater, or even just checking out your stats on the Bungie website will see that Halo 3 definitely deserves this. To break this down, I will put out some questions and see if you can answer them.
It was my JOB to know Halo 3. When I worked for Xbox, Halo 3 was the reason I was hired in the first place. Before the game came out, I had to do a training course specifically on Halo 3.

So, in conclusion, some aspects of Halo 3's systems might have been in other games, but in how many games have they been so spot on and polished almost to the point of perfection?
Again, polish is not innovation. Nobody's arguing that it doesn't do these things well.