I think it's probably between Gun Runner's Arsenal and Lonesome Road. LR ties off the story and should answer alot of questions but we've heard more about GRA and arguably has more in it.
It's difficult to compare these two because they're don't offer the same kind of content. LR is a story driven and traditional DLC that will offer a bit of everything (new perks, challenges, weapons, items, quests etc), whereas GRA is really all about weapons and items. If I had to choose only 1 out of the 2 then I'd go with LR, since its importance in the plot is too significant to ignore.