"The Dive Shop" in VA gets my vote. I live in Silver Spring/Burtonsville, and for me it's worth driving 45 min. Great bunch of guys, they bank 32% Nitrox which is usually always available. They fill my tanks to, or slightly above, rated pressure (short fills suck), and are not afraid to service tech equipment such as double tanks. I go to them for tank inspections, O2 cleaning, gas fills, and they manage Millbrook Quarry, and are cool people overall.
I have recently decided to ban Columbia Scuba due to their customer service (or lack thereof). They won't touch doubles, and look at you like you're from a different planet when you mention the word "Deco"....
Oh yeah, ScubaHut in Glenn Burnie is also a great shop.
Szymon