The game I spent the most time playing online, ever, was SOCOM 2. This was pre-MM and nearly cost me my marriage, lol. I would play until 5 or 6 in the morning.
The best I ever got at an online shooter was at RS:V. This is pretty much where I started to really "meet" people I knew online. Maynard, Twigg, jms and goddamned CrimsonGhost with his fucking C4 grenades. There will never be an online game I have as much fun playing as that one