Michael --
It definitely depends where you are in Fairfax County and how much time you will have. Personally, I'd stay away from Reston National -- overpriced and typically a very slow pace of play. Herndon Centennial is a cheaper option near by that you may enjoy.
Laurel Hill and Westfields are both very good golf courses; Westfields is more walkable, though I've never seen anyone walk it. Laurel Hill has become my favorite area course, it has some unique holes and is a good challenge; it's built on the grounds of the former federal Lorton prison. Westfields is solid, if a bit more conventional.
Two additional recommendations for you:
1. Twin Lakes (down the street from Westfields) is a 36-hole muni, the Oaks is a really strong track, very walkable and much cheaper than the other courses named so far.
2. If you had more time, I'd send you out to Shenandoah Valley Golf Club in Front Royal, about an hour or so drive (depending on where you are). Best value in the DC area.