I have played all the way through Borderlands 2 (except DLC) and I would be hard-pressed to definitively remember a single zone in it, personally.
Fair enough. Personally, the stand-out area for me was the Highlands, with the Sandsea (from the Captain Scarlett piratey DLC), Lynchwood, Opportunity and the dwarf caves whose name I've forgotten (from the Tiny Tina's Bunkers and Badasses DLC) being close runners-up.