I have spend more time on this game then probably any others (include non-shooters!). Battlefield 1942 lets you fight on huge maps with 64 players, and you can even drive planes, tanks, and sometimes ships! The best thing about this game is the mod support, after I got tired of the stocked maps, I continued to play on the desert combat and forgotten hope (What a great mod!!) maps for years!
The original Unreal Tournament is the game that brought me into the multiplayer shooters, it has a frantic pacing and awesome weapon design (love the flak cannon!), the later UT (2003, 2004, and UT 3) didn't grab me nearly as much.
I was late to the party on Halo, I didn't get an XBox until 2004, it took me a while to get used to the controller, but what a great game! The scope of the game was unparalleled, you would come out to a field and there would be a huge battle going on, very impressive for its day!
This is the first Shooter that I really got into, I played Doom and Quake before, but never really liked those, so this game basically made me a fan of the genre! It helps that it is based in the Starwars universe and has light sabers!