Halo 2

They took Halo and somehow made it better. More guns, more levels, more enemies. But that's just the single-player.

The crowning achievement was the addition of online. They took the online they had tested out with Halo 1 and refined into something near perfection. That was my real introduction into online gaming. I played that shit nonstop. And I was so good. I quickly figured out that the human pistols were OP AF. Dual wielding them and actually being good at headshots just FUCKED people. Apparently they eventually nerfed them but I think I stopped playing around that point lol. I had likely moved on to the true addition: Counter-Strike Source.