I've been to plenty of breweries in Loudoun and Fairfax counties. It's hard to just say a beer or brewery is the best, especially since it's a matter of opinion, and there are dozens of them in these two counties alone.

Most of them are very good, but also they rotate their beer selection. The beers that made a brewery your favorite may get rotated out for other things. If you like the vibe of a place that's something that will less likely change, but that's very subjective.

I'm not sure if you're new to the area, but if you are and not just visiting, you could try a few places near your home, then increase your radius until you find something you love, or just frequent your closest places you like to support your "neighborhood" businesses.