Opened in 1972, the RAF Museum fills five hangars with warplanes from the past 100 years: restored WWI fighters, seaplanes, WWII fighters and bombers, and jet aircraft from the Cold War's earliest days, to the Harrier and the F-35.
I've been to a lot of air museums, but the RAF Museum had many planes I'd never seen before, and many that are the only ones in existence. If you're in London, definitely head up to Hendon, especially since you can't beat the entrance price: free.
Unless you drive there, you enter the grounds after a short walk from the Colindale station on the Northern Line. It's about a 15 minute walk.
Here's a wonderfully well-preserved Gloster Meteor, the first operational allied jet. The visual difference between it and the Mosquito adjacent is extreme, especially considering they're from the same era.
The sprawling Bomber Hall (though, I guess it'd have to be), features some iconic and rarely seen aircraft. This display features a Handley Page Halifax salvaged from a lake in Norway after a belly landing.
The beefy Republic P-47 Thunderbolt. It was equipped with eight .50 caliber machine guns, plus this model could carry 2,000 pounds of bombs. Fierce, and the predecessor (namesake-wise, anyway) of the epic A-10.
This Junkers Ju 88 was given to the British by its pilot, who defected and landed at what is now Aberdeen International Airport. The co-pilot was also in on the defection, but the radio operator was not.