The general consensus is that Broken Steel and Point Lookout are the best, Operation: Anchorage and the Pitt are mediocre, and that Zeta is pretty bad. You're not going to get a black and white/yes and no answer from anyone, it's all based entirely on opinion. Here's mine.
Broken Steel: It makes the game open-ended(normally it has a definitive end unfortunately) and allows you to level up to 30 instead of only 20, adding more perks, stronger enemies, plus a few more items, locations, and quests to the mix. Your actions where the game would otherwise end also have noticeable effects once you begin BS. A note though, you can't begin any of the new quests until you clear the game's original main quest line revolving around finding Dad.
Point Lookout: Adds a ferry that will take you to a great pre-war vacation hotspot, Point Lookout, Maryland. It's got it all. Completely open-ended, beaches, swamps, irradiated hillbillies, foul-mouthed ghouls, new shotguns, cults, a Necronomicon knockoff, disembodied organs, amateur surgery, drug abuse...Point Lookout is a completely new place to explore. If that's what you liked about the main game, you'll love it. Beware those irradiated hillbillies though...In spite of their appearances, they're some of the most dangerous enemies in the game. Going to Point Lookout below level 20 is *NOT* recommended.
Operation: Anchorage; The Brotherhood Outcasts found a nice pre-war weapons cache but here's the thing...they need someone with a Pip-Boy(you) to help them gain access to it by completing a wildly embellished simulation of how General Constantine Chase liberated Anchorage, Alaska from Chinese invaders in 2077. It's a joke, honestly. It's very easy because of of the infinite-use, no-cooldown Health Dispensers and Ammo Dispensers within the simulation. Worse, even your first time through it shouldn't take you over 4 hours to complete *EVERYTHING* and there is absolutely zero replay value for the DLC. Do it once and you've done it all. Clearing the simulation also gives you access to two sets of armor that are so strong they make the game laughingly easy. Use at the risk of your own fun.
The Pitt: A leader of a slave uprising in a settlement that was once Pittsburgh, Pennsylvania has come to the Capitol Wasteland in search of outside help to free his people...but something feels a bit off. You get to decide the fate of the slaves and see why slavery sucks so much firsthand, but there's not much else to the DLC. The Pitt itself makes the Capitol Wasteland look like a lush paradise, the trogs make ghouls and super mutants look ordinary. It's not as limited in replay value as Anchorage is, but outside of its story there is very little to it. Upon completion of a ridiculous fetch quest, you get some nice loot but it's not spectacular.
Moterhship Zeta: Oy vey. If you ever thought that Fallout 3 has too much freedom, too many choices, and too much replay value this is the DLC for you. There is no reply value, the entire story is "hurr-durr, aliens got me and I wanna be free." The experience is almost completely linear(with the only choices being which areas you explore from the Engineering Core), the ship itself is dull collection of hallways and rooms that look exactly alike for the most part. The aliens are roughly as strong as the swampfolk in Point Lookout and it takes a few hours to get through the quests to escape the ship, so things become tedious FAST. Bizarrely, it also has the best loot of all DLCs. This, coupled with the fact that the majority of the ship is locked down once you enter the Death Ray Hub: preventing you from gaining numerous unique items you may have missed and getting the Alien Archivist achievement. If you *DO* play Mothership Zeta, make absolutely certain to bring a guide/walkthrough.
Those are my personal opinions mingled with some facts there. I'll trust you can separate the facts(e.g. descriptions of the loot) from the opinions(how much I despise Zeta).
(Expert) - 10 years ago