Calvert-Woodley is my go-to liquor store. Staff (especially Pepe) is great at wine-food pairings. I also pick up wine at Trader Joe's on occasion, and have found some good deals at World Market in Friendship Heights.
I live in the wilds of College Park, so my options are rather limited. Corridor Wine & Spirits in Laurel is good. More locally, we have the Village Pump. They have a better selection of wine and microbrews than your average PG county booze shop.