Halo: Combat Evolved. It's impact on the genre cannot be overestimated. It either introduced or popularized (often by virtue of being the first title to do them right) many mechanics that are now genre standards. One of the most notable examples is the so-called Golden Tripod. Being able to melee opponents or throw grenades without having to cycle through your inventory and formally equip a melee weapon (fists, chainsaw, or whatever) or grenades was something I had yet to experience in an FPS. Halo also had the first truly effective integration of vehicular combat in the genre, whereas in previous shooters I played you either didnt have vehicles, or they were poorly implemented and tacked-on affairs. It popularized the dual analog "left stick to move, right stick to aim" controls pioneered by Alien Resurrection and TimeSplitters the year before. Both the Campaign and multiplayer stage designs were incredible and included a good mix of massive outdoors environments in addition to the smaller indoors arenas. It was as far as I'm aware of the first FPS to have regenerating shields. Finally, you could only carry two weapons at a time instead of an entire arsenal, another thing that was as far as I know a first in the genre. The two-gun limit was interesting in that it forced you to make tough tactical decisions on the fly (Do I go pistol and rockets, or shotgun and sniper, or maybe something else?).
All told, Halo was the most revolutionary FPS since Doom was released in 1993, and many of the things we take for granted today in FPSs we have Halo to thank for. It really was Combat Evolved. Also, let's not forget that it was arguably single-handedly responsible for putting the Xbox on the map. It was attached to around half of all Xbox systems sold by March 31, 2002, and had sold over a million copies in that span, a faster sales pace than any other sixth-gen game at that point. Without Halo, it's likely that the Xbox would have been far less popular than it was, and may have stalled out of the gates and struggled to find its footing, potentially leaving the brand to wither and fade into obscurity. In a way, Master Chief was MS's Mario, a mascot character headlining a game that was responsible for making the console it was on a household name.
So, Halo CE didn't just evolve FPS combat, it built a console brand. Because of this, I rate it among the most important video games ever.
Oh, and it was and still is incredibly fun. I still play the campaign on a regular basis, and I've sunk far more time into it than any other FPS campaign.