I voted Broken Steel, simply because it let you play after the end, plus added some interesting new side quests. Oh, and a GIANT MOTHER F*CKING ROBOT. ....
But I enjoyed The Pitt a lot aswell, that would be my second choice, the moral aspect, as well as the different scenery.
Point Lookout was ok, again had some interesting stuff, but I felt the wild men and tribals were so far overpowered compared to their Wasteland counterparts, it was an obvious way of them trying to put some challenge back into the game. That said, i did enjoy running around Megaton with the Pint Size Slasher mask on

(My charcter at the time was a chinese midget)