Anything on Amerish
Shadespire Farms - as odd as it sounds, it may just be nostalgia as it reminds me of one of the Dutch maps in COD 1. But I love fighting there, Wide open on one side and a slightly cluttered route to the point on the other. Just simplistic and entertaining.
The Bastion - Multiple levels done well.
Lithcorp Secure Mine.
Splitpeak Pass and Auraxis Firearms would be close to favourites as well along with the Ascent and Ravens Landing. Not that its common to have a good fight there. But anyway.