Surprisingly many games I've played.
There are many in some way special, like in Mafia I didn't enjoy gameplay but it has one of greatest stories in any game I know.
Medal of Honor: Frontline was mentioned but not MoH: Allied Assault? The first game in which I ever shot another player online.
Without a clear personal favorite, I'll just give it to one I've put most hours in: vote for the original Battlefield 1942