ARMA 2.
THE most realistic shooter I've ever played. Also it's boring as hell. Running 10 minutes through a forest, getting to a small village, recieving a bullet from an enemy who's a mile away, then waiting 5 minutes until medic arrives and listening to those HORRIBLE VOICES is not for me.
Far Cry 2 deserves a mention too, but the realism was ruined by guards, who respawn as soon as the checkpoint is out of your view, brand new guns, which jam after a couple of gunfights, buddies, who appear out of nowhere just in time to save you, weapon convoys, which drive in circles and continue driving the same route after being attacked, gun storages, which work like vending machines, the fact that you recieve a phone call right after you've done a mission (how the hell can the caller know what I just did?) and the fact that your character doesn't speak a single word during the game and still manages to make friends.
Rainbow Six Vegas was also pretty realistic and actually fun. Take note, ARMA.