Play Headquarters, if you want to get the most XP.
Tips:
Try to play in a full party.
The goal is to capture the HQ everytime (Obviously).
Don't worry about getting alot of kills until you capture the HQ.
Defend kills are what your willing looking for.
Class setup:
Primary: Any Assault or SMG
Secondary: RPG
Semtex or Frag
Stun or Flash
Claymore or Tactical insertion
Flak Jaket
Warlord Pro
Marathon
I have been playing today and average 12K+ Match XP every game.
Most games 20K of Total Match XP
I have also got 36K from challenges in 1 game.