Obviously Broken Steel. Not only it allows to forge a more subsancial aliance with tha BoS, as it exageratedly increases total game time. You don't finish the game on Project Purity, as would happen in the normal game, even with the other DLCs.
Operation Anchorage is really awesome and provides top-quality gear early on. As the Winterized T51-b Power Armor doesn't damage over time, and has got the second best DT (45) in the whole game, losing only to the Enclave Hellfire Armor, and the "common" T51-b, (both have 50 DT), you're totally capable of beating the game using that armor.
On the Other hand, Mothership Zeta provides tons of alien weapons, wich has some mindblowing quality, and are worth thousands of caps. Per unit. Not to mention you can finnaly get aa .44 caliber scopeless, just gotta let Paulson die and loot of itr's corpse. The Paulson's Revolver is the second best revolver in the game, in terms of stats, but it's the only .44 caliber which hasn't got the scope.
Point Lookout is, in my opinion, a bit of a waste, sure, it has a few classic weapons like Double-Barrels and Lever-Action Rifles, but, honestly, is quite boring, the sky's always dark, you're enemies are a few dogs, some mirelurks and a bunch of hillbilly treasure hunters, I mean, c'mon!
Also, last AND least, ther's the Pitt. I'll just say that the only type of person who should buy the Pitt instead of any other DLC on the game, even Point Lookout, is those who would love to be enslaved buy psychopathic,chem crazy, rapist and irradiated assholes. You spend most of the DLC as a slave, and when you finally earn your freedom, you become some errand boy for the Raiders? Something's rotten in that DLC and i'm not talking about the Trogs, the irradated enviroment, or the asswipes who OWN you, oh wait, i do mean that. The only thing worth paying about this particular DLC is the special weapons you get from delivering ingots, but is that really worth getting slaved for? I know I wouldn't take those odds.
Anyway, I think you should buy Broken Steel if you're only buying one, but, if I were you, I'd sell the game and buy the Game Of The Year editon, you'll save a lot of money and have all the DLCs you can ask for (And a couple you shouldn't ask for, but are necessary for the Platinum throphy and a few achievements).