I would have to say that Battlefield 1942 was my favorite. Not the mods, but the original.  I used to play it hours on end and I used to own people at it.  Oh the memories.  

My second choice would have to be Counterstrike, primarily the older versions.  Definately before source...   Probably like 1.3 version.  

counterstrike was a good game but it has way to many cheaters playing it. i use to love the dust2 map

I started years ago playing Castle Wolffenstien.  Played Halo pc for a few years.  Now I play COD4 and Team Fortress 2.

I spend the most time playing TF2.

CoD4 on the PC.

Can't wait for CoD5 next month

Dark Forces 2: Jedi Knight, used to rape people in that.

Or Counterstrike, before CS Soruce I liked Beta 5

BF2 or COD4....

Everquest 2.

Still playing.

team fortress 2, day of defeat source

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Quoted: CoD4 on the PC. Can't wait for CoD5 next month Same here...   I play COD4 almost every night...  It is a blast...   I am looking forward to the WWII action and seeing how well the game play is...

I think COD4, BF 1942, Tribes, and BF 2 are my all time favorites.

COD4 is up there because it absolutely nails the weapon mechanics.  Sure, damage isn't quite believable, but it's right for gameplay.  I've never seen a game that lets you move and shoot as well as COD4 does.  CS:S is completely outclassed.

BF 1942 is up there because, well, we all know why.  Engaging in a submarine duel, then swimming to a friendly landing craft which drops you on a beach, running to a jeep, driving to a hangar, and shooting an enemy pilot trying to taxi his plane out of it, well, what can you say.  Landmark in mixed gaming.  May not be beaten for years, as newer games try to pack so much graphics in that they block themselves from letting you fire a Garand at a guy on a beached battleship.

Tribes is up there because it really originated the team gaming concept.  Sure, prior games had teams, but in Tribes, teams meant something.  If you didn't play as a team, you'd lose.  If nobody guarded the flag, you'd lose.  If nobody looked after your generators, you'd lose.  If nobody went after the enemy's, you'd lose.  If nobody watched the flanks or repaired the turrets, you'd lose.  The map screen was a brilliant invention that really enabled you to see what your team was doing, and respond to it.  Possibly the very first Blue Force Tracker in the world.  Never mind the zaniness of using jet packs to propel yourself around an enormous 3D battlespace at terrific velocity, and still find room for almost hand-to-hand brawls.

Note the great voice system; Tribes had several voices with dozens of shortcut activated phrases each.  Decent players could literally converse with each other using the system.

www.youtube.com/watch?v=yjbOUmSfPe8&feature=related

And BF 2 gets the nod because of its command system.  When officers are asked what modern command is like, the answer will be completely recognizable to BF 2 players.  They are the guardian angels of the battlefield.  When you run low on ammo, when you need transportation, when you're hurt, when you're confused, when you're trapped, when you think the enemy is breaking through, the commander is there to save your ass, with artillery, information, guidance, or maybe just a speeding van packed with C4.