Well, DLC 1 is a whole lot of dried up seas and what not so mostly it's kind of bland to look at but Hayter's Folly (a sub-area you quest through) is pretty cool. Washburn Refinery has some kind of graphics problems or something because it always looks weird on my TV and gives me a headache.
DLC 2 is mostly desert as well. None of the areas are all that cool but I will say that the end part of the Forge (a giant structure where a key story battle is fought) is probably the second best piece of level design I've seen in this game right after Opportunity.
DLC 3 takes place in a mostly swampy locale that is reminiscent of Jakobs Cove from BL1. Of the three DLCs I'd say DLC 3 probably has the best atmosphere.