Whole Foods P St is the best. Also Conn Ave Wine & Liquor in Dupont Circle, D'Vines in Columbia Heights. Across the river in Arlington, check out Arrowine and Westover Market, and in Falls Church, Dominion Wine and Beer.
Schneider's has some stuff sometimes, can't echo Whole Foods on P St enough...Westover, ehh only if you like paying 25% more...Arrow is a small shop, but has a good selection as well.
Agreed on Westover. Lots of quantity but not much quality and very expensive. Although it's only 5 minutes from Arrowine, and there's a Lost Dog right across the street from Westover that also has a pretty nice selection. You could theoretically hit all three in 30 minutes.