This except for SP it would be CrysisRewold said:Battlefield 2. Two IRL friends in a locked squad with snipers running around wrecking havoc. And no, we didn't just sit on a hill. Usually we took a buggy and drove in the middle of the battle.
CoD: United Offensive with huge map and 60 players. That was really cool.
On single player side I'd say Bad Company 1 and TimeSplitters: Future Perfect! Awesome and fun campaigns cleared more than once.