UNSTOPPABLE!!!
First off, I've easily spent over $130 on this game with all of its material including the game guide and I don't regret a single penny of it so for anyone who, for some stupid reason, doesn't already have this game, $60 is a Godd**mn steal. I mean, seriously, the only reason that I can accept for someone not having this game already is that they were too young to buy it last year because it's rated M. Anyway...
BROKEN STEEL is definitely the best because, not only does it offer more new weapons and armor than any other, it also expands the main story line, adds ten extra potential levels and new perks, and allows you to continue your game after the story is over.
POINT LOOKOUT is my second favorite simply because the swampland environment is awesome. It also adds more new quests than any other DLC and they're really fun and interesting. It doesn't bring much in the way of good new equipment except for the lever-action rifle which makes excellent use of your very abundant 10MM ammo.
OPERATION ANCHORAGE, I have to say is my third choice. I know, most people say it's the worst and for the most part they're right. BUT... despite having only mediocre new quests and only offering one new weapon and one new set of armor to the main game, that one new weapon (Gauss Rifle) just happens to be the best gun in the entire game. AND the one new set of armor (Chinese Stealth Armor) just happens to be the second best armor in the game behind the Enclave Hellfire Armor from Broken Steel.
THE PITT is also a worthy add-on because it introduces the Auto Axe, which isn't as deadly as it should be but, is still awesome. Plus it offers some interesting new quests and a nice new area to explore.
MOTHERSHIP ZETA proved to be the most tedious although it's still very interesting and compelling. It brings some unique quests to the table along with a big handful of awesome alien weaponry. Perhaps the best item that you can bring home with you is Alien Biogel which heals you better than stimpacks.
I have over 300 hours on only two playthroughs and I can safely say that Fallout 3 is my favorite game of all time. If you've never played it then I feel utterly sorry for you. For more info on gameplay features and mechanics checkout my review on the original release.