I have to go with Tiny Tina, it has everything. Explosions, Torgue, Tina, Moxxi, and Fun. Also secret celebration of hitting next Karma level. I dont consider the Headhunters big enough to even compare to any of the 4 Season Pass DLC's, also this poll would become somewhat invalid if they announce a Moon Base DLC or other big DLC.
"If your going to ask someone to save the world, make sure they like it the way it is" https://soundcloud.com/shake-like-e