I forgot to mention the Half-Life 1 mod Hostile Intent. It's one of the more realistic tatical shooters, to the point that you literally have to check in with your teammates on the radio to see if they're still alive. You can play as NATO (catch-all for The West, in fact depending on the map the NATO team can be anybody ranging from Navy SEALs to the local SWAT team) or Tango (criminals, terrorists, or just enemy soldiers). Despite the old graphics, the sensation of firing your weapon comes closer to the real thing than any other game, but you have to play it to understand.
A poorly coordinated attacking team stands virtually no chance, and since NATO is usually attacking, they are the harder team to play as.
I personally enjoy the variety of playing as the bad guy in a modern tactical shooter, which is usually not allowed. You can be an arms smuggler hiding in the corner of your warehouse with a suppressed MAC-11, a Colombian drug lord crouched in the bushes with an AK, or a Russian nationalist with a AKS-74U fighting for the Motherland, to name just a few.