Probably Mass Effect 3's Citadel DLC. While I wish it would have been included already in game when it shipped out, the whole DLC was extremely enjoyable, light-hearted and hilarious to see everybody interact at Shepard's pad. It was a great send-off to a great trilogy.

Close behind is Left Behind. I loved exploration Ellie and Riley's relationship and the reveal I was hopping would be true.

GTA4 episodes. it actually made the gameplay better and both stories were better than the main GTA4 proper...not bad

Im the opposite, i dont like the stand alone dlc.  I like it when the dlc is integrated and expands the main story of the game.  Stand alone dlc usually is a lot more isolated, generally ends up feeling like a small arcade game using the games assets.  

I hardly every buy DLC. I guess the Batman Arkham City DLC was the best I've bought.

It's somewhat a tie between the Shadow Broker for Mass Effect 2 and Left Behind for The last of Us. I favor the latter a bit more.

The only DLC I have ever bought was NSLU

Hmm, I dont buy DLC or expansions very often these days but the best one I bought in the olden times was Command and Conquer: Zero Hour and man, did I play it like crazy!

Horse armor

Either Left Behind from TLOU or Undead Nightmare from RDR. They serve different purposes but they are both incredibly awesome!

Mass effect 2/3 by a light year,

oh and because its the only dlc ive ever bought