Whole Foods Market has the most consistantly good produce of all of the supermarkets in DC area. Many items are organic (if that's your thing), but for me, it's all about a large selection of very good quality produce. Also has a good selection of packaged produce such as nuts and dried fruit. The DC location on P street has excellent meats and fish. Very helpful personel. Downside is that it is pricy.
Pros: Variety, Quality