I would say the metro, it's not very far. May not be "best" to you, depends what is important to you and how close you are staying to a metro stop, also. There aren't really that many choices, I wouldn't take the bus.
DCA is almost in Alexandria. Alexandria covers a big area. So exactly where you are going makes a difference. I expect a taxi could be your best option, as it will give you door-to-door service. It will not be expensive.
As others said the Metro is the best option if you are staying near one of the Alexandria stations. Taxis will be 15-20$ or more depending on traffic. They are the most convenient and you are very close if going to old town.