Will be for one day in Washington DC

I will be for only one day in Washington DC. I am planning to take the bike tour (10am-1PM) to see as much as possible. After that I need to pick only *one* museum.
Please let me know which museum is a "must"?
Which one is rated highest?
Which one I will regret the most if I miss?
I know many of you would say: "that depends on what you want or what you like". See, I like art, science, and history too, so I like everything which is good, interesting and high quality. So the theme does not matter. Thank you!

How about this:

1:00-2:30 Visit the National Gallery of Art. Look at their website before arriving to pick a few galleries or exhibitions of particular interest. If it's still showing when you're there, consider the Afghanistan Archaeological Treasures show.
http://www.nga.gov
http://www.nga.gov/exhibitions/afghanist...

2:30-3:00 Have a late lunch (minus the lunchtime crowds) at an outdoor table at the outdoor sculpture garden adjacent to the National Gallery of Art. Walk around the garden.
http://www.nga.gov/pdf/pavilion_cafe_men...
http://www.nga.gov/exhibitions/sculpture...

3:00-5:00 Walk across the Mall to the National Air & Space Museum. See an IMAX film. Be sure to see the Wright Brothers Flyer, the Apollo 11 capsule, and the Spirt of St. Louis. Stop at the gift shop for a souvenir of your day.
http://www.nasm.si.edu

Whatever musuem you pick - go to the information desk and ask for the short tour or highlight map. You are not the first person to have a short time frame - the Art musuems even have them broken down to if you have 30 min or 60 min.

Anyone of the Smithsonians (If you have ever been to any other musuems space and air exhibit - the national one is not much different.

I don't know about the bike tour that you are talking about - you would be better off taking a Tram tour to get to all the monuments and get on and get off when you want or just drive by some.

Will since the American History won't be open until November, you can see some of the most popular items from that museum at the Air & Space museum.

I love the museum of natural history! We go quite often and still find new things to see! Air & Space is ho-hum for me...I do love the Air & Space museum in Chantilly, VA near Dulles, but the one in the District isn't that thrilling to me anymore.

The museums are so close to each other that you could give yourself an allotted time at both and do a quick run through on the big things you would like to see.

Do a little internet research on the exhibits and determine which will best float your boat!

The air and space museum is the top must in my book. If that's not your thing then the museum of natural history.