1: Halo
Halo is an obvious choice for one of the 00s best games. It served as proof that consoles could sell first person shooters. Even more so, it proved that consoles could in fact thrive and survive with first person shooters. On top of it all, Halo delivered a story that borrowed heavily from various high- and hard-sci-fi sources, mixing it down for the masses without losing the source material’s potency. Also, Halo served as the blueprint for shooter controls and game design unsurpassed for years to come.
Love it or hate it, Halo rewrote the rules of the gaming landscape, big time. And it aged comparatively well. The AI is still outstanding. The shooting still solid and polished like few other games ever managed. The level design might be a bit hit and miss, but that never really mattered. Even a decade later, Halo still stands the test of time.