I think New Vegas was possibly the best in the series, and one of the few games to have really good DLC.
Agreed, with the exception of Honest Hearts
which was unoriginal, boring in many places and filled with pretty useless items like tomahawks and semi-auto guns that pale in comparison to what you have access to at Gun Runners once you've broken the casino banks.
Despite being annoyingly difficult and with a quite a bit of backtracking, I think Dead Money
is one of the best RPG DLC's to have ever been released. In fact it's so good, that Bethesda could probably make a whole game based upon the area and the characters introduced in DM, if they were so inclined.