What is your favorite setup to use in MW2?

First I like to use either a SMG or a LMG.  The SMG for small fast paced maps and the LMG for other maps.  The lightweight SMG guns are pulled up in the firing position from running much faster than the LMG, but I use the LMG when I want a huge clip to blow hordes of enemies away.  I also snipe with the SMG guns by tapping the fire button so as not to make the gun spray wildly.

Perks & attachments:
Marathon is a nice one.
I almost always use stopping power, I like to save bullets.
The Commando perk is a nice new addition.
I use the steady aim with the ACOG scope or Thermal scope on a LMG for multi-range action.  I don't aim for close ones and the thermal gives you an easy target when enemies are in view.
I use red dot or holographic on the SMG guns most of the time.
Grenades until I unlock claymore mines.
I got one slot made up for a stealth character and one for a sniper which I haven't tried using yet.

Here's a list for all of the perks and what they do along with the pro version.

http://www.themodernwarfare2.com/mw2/multiplayer/perks/

So if I'm playing a game with you and I see the snow fields on my radar then I know you're close.