Operation Flashpoint is by far... one of if not the best military first person shooters to date.
Although the graphics are lagging slightly by current standards (remedy is coming in the form of OFP2), it has the most modifications, most customization, and most realistic situations.
For those of you who haven't played it, Operation Flashpoint allows you to occupy positions in dozens of vehicles, play on maps that will literally take you an hour or two to cross on foot, utilize various weapons, and take part in coordinated operations that go far beyond 'take the flag'.
And that is simply in it's unedited form! There are HUNDREDS of aircraft and vehicles custom made for the game, as well as new maps and entire themed packs such as WWII, Vietnam, Golan Heights, or desert combat. The game also includes the ability to create your own missions in depth and organized detail.
What other game allows you to take your squad of three men to the edge of a road to mine it, duck away while an enemy patrol comes along, watch the enemy convoy explode then call in choppers to medivac wounded or strike targets you specify. All the while knowing that enemy reenforcements are on the way and that you need to make it to an LZ ASAP. Of course, if the door gunner of the Blackhawk is killed, you can grab his 60 and let the enemy have it.
No other game that I have ever played ever offered so much. And unlimited replayability.