I've visited only a limited number of air museums. Here is my complete list:
First rank-
Duxford, Shuttleworth, RAF Hendon, Air & Space at Le Bourget, USAF Museum at Dayton, Natl Air & Space at Washington [NASM], also the restoration faciility of NASM which is being replaced by the new museum at IAD Dec 2003 and I hope it's as good.
Second rank-
Brussels, air section of Science Museum London, air section of Reichs Museum Munich.
Third rank-
Naval Air Museum Pensacola, Florida.