I was just there in the middle of May and did a lot of research into where to find good beer. Here's what I found.
Close to the Metro stations:
The Big Hunt - Really close to the Dupont Circle Metro station (across the street). Has a good draft selection, almost zero bottles. Kind of a dive bar.
Pizzeria Paradiso - About a 2-3 minute walk from Dupont Circle Metro station. Good pizza and a nice selection of draft and bottled beer. Pretty good Belgian selection.
Bier Baron - A little farther from Dupont Circle. Maybe 10-15 minute walk. Huge bottle selection (Only place I found Great Divide beers). 15 or so draft selections. I went twice and had a great bartender the first time and a sub-par one the second.
Connecticut Avenue Liquor - Awesome Liquor/Beer store about 3 minutes from Dupont Circle. Big selection of bottles. DFH, Surly, Ballast Point and many others. Good Belgian selection as well. Really nice people too. Went there twice and he remembered me and was quite knowledgable.
Farther from Metro Stations:
These ones are probably 15-30 minute walks from the closest Metro.
The Black Squirrel - This place was awesome! 2 or 3 levels. 15-20 taps on each level plus a few bottles. I had Widmer, Brewdog, Ithaca and Southern Tier here. Nice and helpful bartenders as well.
Churckey - If you've looked for beer in DC I'm sure you've found this place. It's pretty sweet. Around 30 beers on tap and like 500 in bottles. Great selection. To be honest, the atmosphere is a little pretentious (as are the bartenders), but it's not that big a deal. They have 4 oz (ish) pours of all draft beers too which is awesome! That way you can try a lot more of the draft selections without getting demolished.
One other suggestion for bottles to buy would be Whole Foods. I found Allagash Confluence (SOOOOOO awesome) and Weyerbacher Double Simcoe there (among other tasty choices).
Hope this helps. There is a lot of great beer in DC. Enjoy!!