MW2.
Once you got to grips with the game you could get some huge killing sprees going, and overall the game felt very rewarding, especially trying to unlock all the unique callsigns.
As Montana mentioned, there was a variety to the gameplay. I had load outs made for different levels, so you would be switching from a sniper class with a heartbeat sensor on one map, to a shotgun with marathon for the very next game. For the most part, most set ups could be viable on most maps, so it came down to a choice of how you wanted to play.
You also had enough health that you could evade and avoid enemy fire if you reacted quickly enough, leading to some hilarious situations where you enter a room with 3 enemy's, all trying to shoot you, and you wipe them all with a shotgun.
I was really hoping MW3 would just be an improvement of MW2, but so far the awful maps and Lower health (I think?) have really ruined it for me. It can be fun, just not on the same level as MW2.
