In terms of sheer development activity/construction, the 14th St. corridor is absolutely blowing up. The area between Logan Circle and Columbia Heights is filling up with all sports of 6-12 story condos. While these hoods are definitely established, and have been on the gentrifiers' radar for quite some time (Logan is tired these days), the level of construction is impressive.
Personally, I would put bets on Eckington; equidistant to U and H streets, next to the NY Ave Metro and NoMa, across North Cap from Bloomingdale, walking distance to Union Station, Downtown, and Cap Hill. It certainly has alot of potential. Edgewood and Brookland, and, basically, all of those inner-NE areas near the Brookland and Rhode Island Ave Metros are sleepy, largely undeveloped, and offer lots of potential.