Depends if you want to fight the crowds or not.
If you are looking for a diner-type place to sit and have a good meal, casual, and friendly, the I would recommend Krupin's Delicatessen. I think it now actually called "Morty's" and it on Wisconsin Avenue in Tenleytown. There is always parking, and usually not a long wait. The food is worth the wait though. BUT, you have to like a loud place with pink and baby blue vinyl.
If you are looking for something a little nicer, you will probably need a reservation but there is a new place called "Meridian" that is at the corner of Georgia and Missouri that I hear has a great brunch. It is kind of trendy.
Where are you staying? That can help us more