Worst DLC has got to be mothership Zeta. Like Anchorage, but without hilariously broken loot to make up for it. OP enemies, no relevance to the rest of the world, and a nasty tendency to break and ruin your savegames.
Best, on the other hand, is the DLC adventures in New Vegas; all four represent an excellent level of storytelling, worldbuilding, and dialogue. They don't just give players more buildings full of super mutants to gib, they tell an entire secondary plotline. They aren't necessary to enjoy the plot of the game itself, and with the exception of the last one, they all stand alone so if you don't want (for example) an 8.5 hour art-deco survival horror quest where the entire world actively loathes you and your every move is watched by someone who can kill you with a push of a button if you disobey, you don't have to buy it and 1/2 of the DLC is still accessible very good.