Personally, I like all of the DLC. I'll expand on a few things that the others have left out.
Witch Hunt for Origins kind of expands on Morrigan and her Eluvian.
Lelliana's Song gives you her back story.
Warden's Keep is also interesting and gives us the first real look into the darker world of the Wardens.
Exiled Prince for 2 gives us some interesting and amusing banter from Varric in Inquisition.
Legacy does give you a lot more context for Inquisition as said before. It also gives you the chance to make yourself a cool weapon.
A lot of the DLCs seem to give you some cool stuff. Weapons/Armor/Schematics(Inquisition only). For those reasons, I find them worth playing. But it really depends on if you're more interested in action or in the story. For story, they're all worth it. For action, you get stuff to help you out. Therefore, they're all worth it, imo.
