How about Bull Run? Just kidding.
I personally vote RFK, but that's also because it's walking distance from my house.
Hey Seth, the next time you do one of these Virgin Fest thingies, have you thought about narrowing it down to a Coachella East kind of thing? I know you need the draw of big bands and names, but I'm sure that you could pull it off. In my mind, this is the kind of lineup I would imagine (and by next year, a lot of these bands would be supporting new records or about to support them):
Radiohead, Bjork, the newly-reformed Smashing Pumpkins, Blur, The Stooges (who have yet to play DC), the Shins, Queens of the Stone Age, Interpol, and then you can round it out with any other bands that are name-drawers at that particular moment. I know you have to think about commercial appeal...but Radiohead is a huge name and that would do it.
Of course the dance element will be good too: DJ Shadow, Daft Punk, Aphex Twin, Squarepusher...whatever. But if you narrow it down to a certain niche, you'll give Coachella a run for the money and you'll certainly get people traveling from all over the place for it. I can't imagine Branson wouldn't want to put the Virgin name on something like that.