Carlos Santucci deli in Silver Spring has amazing cheesesteaks, cold cuts and other sandwhiches. The Broiler in Arlington, VA does amazing cheesesteaks as well (I actually found out about them here on Reddit). Paisano’s (multiple locations) has great sandwhiches as well. I would definitely rank Carlos Santucci as my number one, though. Honorable Mentions: Zburger and Charleys.

Editing to say I re-read the title and realized you specifically were looking in DC. But hopefully these recommendations will help you for the greater DC area as well.