As a person who bought all the main DLC (not the gun runners, or courier's stash) I'd have to say the best are as follows
1:Old world blues is by far the best. You get some of *the* best humor I've ever seen in a video game, a great adventurous land scape to mess around in, tons of easter eggs, really cool new weapons, armors, and quests, and a house that takes two (as apposed to the four, and they are far closer together, you just shoot a special gun to get back to the DLC area, and you're spawned right in front of the house's door, trust me, I would have payed the price for this alone). This house is ,by far, one of the coolest things ever. It's got a cool science-ish theme (but still kind of wasteland-ish) has really, really cool features.
An auto doc that can change hair, and do cosmetic surgery, as well as re-do your traits (only once though), heal you, and fix broken limbs. A sink, which you can get water from.
A farm, which can grow a lot of useful plants.
A seed-grower...thing (which can convert all food mater into a green paste that can be transformed into a lot of other things at a camp fire).
A book shreder which can take books, and make them blank (you can then make skill BOOKS [not magazines] out of them, albeit only once).
A shop.
A little robot that takes coffee mugs and breaks them down into crafting items.
A toaster who does the same thing the robot does, but with toasters (he can also super heat a new weapon in the DLC).
Two light switches, which can turn the lights on, or off, and has two extra light modes:"Smart" which gives the place a green tint, and adds +2 to a few SPECIAL skills for a couple of in-game hours, and "mood" which gives the place a red tint, and does the same thing, but with different SPECIAL skills.
All of these have very, very cool personalities (which can even be turned off, if you don't like them, which means that you can get right to the functions of them, without conversation).
It's also got a lot of throw backs to Christine and Elija.
Lonesome road:My second pick, adds a LOT of back story to the courier, really cool weapons, nice armor (including a courier's trench coat, which has a different symbol on the back, that changes depending on who the player sides with) a great space (which is super destroyed and liner) and, best of all, it makes ED-E the best companion out there. He becomes tougher, does more damage, has built-in work and ammo benches, and can produce Satchel charges (which are basically super strong land mines), flamer fuel, and two types of energy cells every day.
Honest hearts:Has a totally untouched game zone, which actually rains! It's got some neat armor and weapons and all, but the real draw is the story of how Joshua Gram (Ex 2nd in command of the legion) attempts to redeem himself through his cathartic fueled quest to save two tribes (genuine native-american themed) from the destruction another (more powerful) tribe (seeking to join the legion).
Dead money:Has a very "spooky" theme to it, and is the most serious of the DLCs. It tells the story of the courier, and his misfit team of fellow prisoners as they attempt to break into an old Casino vault, lest they die. These are some of the most interesting characters in the game, and include a brother hood of steel members named Christine, who's had her vocal cords cut out, forcing players to communicate via hand signals (a high perception is VERY helpful in understanding).
Dean Domino, an old-world ghoul who used to be a famous singer, and has since become very savvy with the area himself, he's looking out for himself, and only himself.
Dog/God:A nightkin who's suffering from multiple personality disorder, one being a blind follower of Elija, who is willing to do anything for him, and is incredibly stupid, while the other (god) is very, very smart, and hates Elija with all his being. The player chooses which personality the character uses, and it can be switched in/out throughout the DLC.
All these DLCs are very, very good, and the devs have nailed it all so well.
