Since I am not into multi family, I am only interested in individual residential bargains, but there areas should also offer bargains on multis.

Prince William County: 20109, 22193, 22191, 22193 …, in general, Manassas, Woodbridge are storm areas.
Loudoun County: 20164, 20176, 20175, …
Fairfax county: 20170, 20151, 22015, …

The above are hard-hit areas with lots of middle to lower income families. However, if you want to see the wealthy belts, follow these corridors:

I-66 corridor in VA
I-267 corridor in VA
I-270 corridor in MD (along 355 as well)
Route 7 corridor in VA
Route 50 corridor in VA

Most expensive areas are:
Great falls, VA, Potomac, MD, McLean, VA, Bethesda, MD, Chevy Chase, MD.

Second tier most expensive: Arlington, VA, Alexandria VA, Vienna, VA, Reston VA, Rockville, MD, Clarksville, MD.

Third tier (mostly middle income): Herndon VA, Chantilly VA, Dulles, VA, Germantown, MD, Gaithersburg, MD, Clarksburg MD.

Forth tier ? the above listed zip codes covered them, the forth tier locations are heavily hit with subprime and foreclosures. But they are not slums, they are up and coming locations more newly developed and housing prices are generally lower, local income families are generally living in the forth tier locations. I bet 10-15 years later, fourth tier locations will get as expensive as upper tier locations.