Well, Operation Flashpoint gets my vote for the best, especially with the ease of creating your own scenarios.
But i remember playing the origional rainnbow 6 and ghost recon too I believe in coop and I believe the newer releases all still have coop as well.
That was fun, but....Swat 4 blows em away for coop action. Especially if you load up one of the realism mods out there for it, Swat 4 coop is just absolutely phenominal.
Also for coop, consider Bioware's 2.4D (2 and 4/10ths dimential
) crpgs such as Baldurs Gate 2 if you're into story and roleplaying. (Diablo 2 and Dungeon Siege can work but are reallly more pure action and super light on story elements, especially for coop!
Also consider Bioware's Never Winter Nights fan made modules many of which are designed for coop play for 2-4 players and range from decent to incredible.
I'm sure there's more i just can't think of em atm.