Not trying to sound cryptic, elitist, or anything else of that sort but that's really all you need. I think both unbelragazzo (and correct me if I am wrong) are trying to give you the best information possible, and avoid giving you anything superfluous.
You can check out Nieman Marcus, Brooks Brothers, etc etc etc but again, you can do that anywhere.
If you want the best tailor in DC, go to William Field. He is not cheap, but brilliant. Keep in mind if you are having something bespoke made, you're gonna have to return relatively shortly for fittings.
If you want the best bang for your buck, I say get MTM Samuelsohn made at WM Fox
If you want to shop for shoes go to Sky Valet, or some beautiful leather accessories go to Sterling Burke
We can offer other suggestions if we know more about what you are looking for: what are you looking to purchase? how much do you want to spend? Are you looking for one great suit or a few items? etc etc etc.