I will be visiting this weekend and wanted to know which metro line do I take from Ronald Reagan Airport to get to the hotel?
Take the Yellow line from Reagan National toward Greenbelt. Change trains at L'Enfant Plaza and get on the Green line toward Branch Avenue. The state of Maryland runs a bus service to National Harbor from Branch Avenue Station. I believe there is also Patomac Riverboat that takes you to National Harbor as well if you feel adventurous. Happy travels.
You would be better off catching Uber as the metro is not near. I also know that they have a shuttle that picks you up at the King Street Metro but you can confirm by visiting the National Harbor website.
Wicket52 did a great job in answering that question. Check it out. Chelley C is also right in saying that the metro isn't anywhere near the hotel. What I have to offer is that the Residence Inn National Harbor isn't all that far from Reagan Airport and cab fare might not be too bad. (or Uber) I'm pretty frugal and I would never take a cab from Dulles Airport or BWI to get to National Harbor, but Reagan Airport is no more than 8 miles to the hotel.
There is also the possibility that there is a shuttle to the National Harbor Convention Center that is right across the street from the Residence Inn. just saying.
Get an answer from Residence Inn by Marriott National Harbor Washington, DC Area staff and past guests.