Battlefield 1942, with all its mods and teamwork with no unlocks or reward systems, just pure cooperative gameplay. The graphics are dated now, and I haven't played in years, but I'll always remember it as the most fun I've ever had in a game.
Nothing like being completely pinned down by tiger tanks and you hit F2 to request reinforcements for the 20th time and suddenly you hear the hot rod engine of a British Spitfire fighter plane roaring into a dive and BOOM, no more tiger. "Roger that roger that!... Pilot dips his wings as he flys off into the El Alamein sky.