Its easy for me to nominate MW2, mainly because I have it and like it.
But we have Me, Mohawk, Blitzkrieg, fatguy(mohawks brother), Shazz, ZD, and another guy name Sup who is friends with Mo and his Bro. We usually can get a group together each night to team up.
The diff with MW2 is you have to join Parties, exactly like xbox. So once you get 6 people (the amount of one team in HQ) you can click play game and it finds 6 people right away to play you. Same concept with 1 person, or 8 people. It will just find the extra amount of people you need to play.
Nice because there is never a time your playing 1v1 to get others to join, but bad because everyone doesn't have one main server to join and play.