People keep saying Tiny Tina's Assault on Dragons Keep. Goes to show again that I'm the odd duck out since I didn't like the DLC apart from the ending.
Anyway:
Secret Armory of General Knoxx (BL1)
Mister Torgue's Campaign of Carnage, Captain Scarlet And Her Pirate Booty (BL2)
Dead Money, Old World Blues, Lonesome Road (F:NV)
The Pitt, Point Lookout (F3)
Outside big content DLC like those mentioned. I really am not a fan of it. Even if my main in BL2 is DLC.