If you're an aviation enthusiast who is visiting England, then the Imperial War Museum at Duxford should definitely be high on your list of places to visit. If your Significant Other will only let you visit one aviation museum then you're probably better off going to the Royal Air Force museum at Hendon, which is conveniently located in North London and has a somewhat superior collection of aircraft. However, if you have the time to get up to Duxford, which is near Cambridge, then not only can you visit Cambridge, but if you time things right then you can visit the Imperial War Museum and see one of the airshows at Duxford.
Duxford has an excellent selection of aircraft, including quite a few which you won't see at Hendon; however, Hendon definitely has better facilities than Duxford. It's true that Duxford has the purpose-build and award-winning American Air Museum, which contains about the best selection of world war two and later American military aircraft that you'll see outside America. However, the other main hangars are rather cramped and some of them are definitely showing their age; and sadly quite a few large historic aircraft are kept outside, exposed to the elements.
At the time I visited, there were around 180 aircraft spread around the different hangars. Apart from the American collection, there are some British piston-powered and jet-powered airliners, including the first jet airliner, the Comet, and an early test model Concorde, which you can walk through. There's also a good selection of early British military jets. There are German world-war two aircraft like a Messerschmitt Bf109, a Junkers Ju-52 and a Fieseler Storch. Finally there are weapons, such as Barnes-Wallis' bouncing bomb used during the famous "dam busters" raids in the Ruhr valley, and a German Fritz-X radio-controlled bomb, predecessor to modern smart bombs.