I gotta say, for solo, in my opinion, definitely Domination, or Ground War domination. You can get quite a few kills doing this.
For a team, spawn-trapping in Demolition gets you a LOT of kills.
[nomedia="
https://www.youtube.com/watch?v=NBrQmBciGfE"]YouTube- Call of Duty Modern Warfare 2 Demolition Tips on Sub Base (Gameplay Videos & Commentary)[/nomedia] This might help you.