I highly recommend Aikido of Northern VA but I am a bit biased
You have 2 others to choose from in Arlington, one in Clarendon (Iwama) and the other at TJ community center (USAF).
Jimmy Sorrentino's dojo is very convenient to the metro and is supported (same organization) by the Shobukan dojo (Saotome Sensei's official home dojo - check the schedule as to when he is there cause he lives full time in Florida ) over the river and also convenient to metro.
Garrett