Halo: Combat Evolved. The Silent Cartographer. The level was a whole fucking island. Nothing amazing now, but at the time it blew my mind.
If you can't remember how amazing this game looked in 2001, and how much it was ahead of everything else (including what was then current on the PC), then you're too young to criticise it now based on less than impressive sequels.
Halo was the reason you had an (original) XBox. Sure, I love my weird and interesting games (that some other people might call 'art games', as if 'art' only describes a subset of games, where really it describes the entire medium), but Halo got me into properly into console gaming. CoD is maybe the same sort of thing for a new generation; I don't have the same level of affection for it but do like a bit of Zombies on Kino Der Toten.