Ledley by a huge margin for me, given his whole career to us and as someone mentioned, given his KNEES to us! Anyone else in his condition would have probably retired by now, he chooses to go through the pain barrier to play for us. I'd personally give him an on going contract until he decided to retire, even if we do only get 10-15 games a season from him, my loyalty to the man is too great, plus them 10-15 games would mean our chances of winning improve drastically!
Special mentions would of course go to Bale, Benny, Modric (when he's not being a twat!) and Daws.
I think give it a couple of years and Sandro, Kaboul, Rafa and Walker will also be in with a great shout.