The map pool in MW2 (2009) is probably the best in COD history (Terminal, Highrise, Karachi, Favela, etc). The gunplay was also amazing.
I loved the killed streaks, bodying people with the AC130 and the chopper gunner was
Part of the reason why people loved MW2 (2009) was because everybody was playing it. It was peak COD in terms of popularity, so everybody had friends playing it. The social aspect was on another level. I still remember playing 6v6 private match SnD among friends.
As ridiculous as it sounds, so many things were overpowered (one man army, noobtubes, danger close, commando pro, chopper gunner, AC130, etc.) that it balanced itself out.
MW2 (2009) might be the most fun I've had in any COD title.