I can only play for couple hours a day and about 5 days a week, but I can easily say that I put the most hours on BattleField 3 (32 days), nothing can top that for MP in my opinion that is!
The following 2 users say thank you to Lethal Hack for this useful post:
Call of duty 4 to be honest, went to tenth on like 3 accounts and still play it now and again now.
Second to that would be World at War and third would be Metal Gear Solid 4.
The following user thanked Richard. for this useful post: