Battlefield 2.
(It's not a cod clone, or so you may think)
It took the mechanics of BF1942, added to them, took the gameplay to the modern era (something new for 2005) added new things like ironsights for guns, and unlock system for persistent online play, invented a squad system, and added things that allowed modders to make truly fantastic conversions for the game.
You can now even play with up to 128 players on a single server in Project Reality.
Its sequel of the year, all years.