The free museum is dedicated to the history of aviation and contains a huge collection of engines, aircraft, and missiles.
Many of the aircraft are incredibly rare, housing the last remaining Boulton Paul Defiant in the world, and one of only two surviving Vickers Wellingtons. The attraction hosts dozens of events each year including the extremely popular RAF Cosford Air Show in June.
St Fagans National Museum of History, The National Railway Museum and Durham Cathedral were also among the UK's best free days, according to the survey.
The top-rated free attraction is Cardiff's St Fagans National Museum of History, which achieved a customer score of 94 per cent in a survey of almost 7,000 people by consumer group Which?
The open-air museum offers a "fascinating glimpse into history" and features 40 reconstructed buildings including a working forge, a school, chapel and Workmen's Institute, while practical exhibits involve demonstrations of blacksmithing, pottery and weaving.
The National Railway Museum in York, which displays more than a hundred locomotives including the Mallard, a working replica of George Stephenson's Rocket and the only Shinkansen bullet train outside of Japan, tied with the Royal Air Force (RAF) Museum in Cosford with a score of 91 per cent.
Other highly-rated free museums and galleries are the National Museum of Scotland (90 per cent), the Cardiff National Museum (87 per cent), Kelvingrove Art Gallery and Museum in Glasgow (87 per cent), Royal Air Force Museum, London (87 per cent), Walker Art Gallery in Liverpool (87 per cent), Oxford University Museum of Natural History (86 per cent) and the Scottish National Galleries (86 per cent).
The watchdog also asked almost 3,000 people to rank their favourite historic attractions, rating them on nine criteria including accessibility, the quality of food and drink available and lack of crowds.
Durham Cathedral, once described by Bill Bryson as "the best cathedral on planet Earth", topped the ranking with a score of 86 per cent.
St David's Cathedral also rated highly on 80 per cent, followed by Chichester with a score of 77 per cent.
The Houses of Parliament followed on 76 per cent, with any UK resident able to avoid paying around £25 a head by contacting their MP, or a member of the House of Lords, to request a free 75-minute guided tour.
Other highly-rated free historic attractions include Bury St Edmunds Abbey, which received a customer score of 73 per cent, Somerset House (71 per cent) and Bristol Cathedral (68 per cent).
Which? Travel editor Rory Boland said: "As the cost of living crisis continues to be a strain on household budgets, finding cheap or free days out has never been more important. Fortunately the UK has a number of fantastic museums, galleries and other attractions which offer free entry and hours of fun.
"From the world's largest railway museum, to the Houses of Parliament, Which?'s rankings of the UK's best free days out has plenty to inspire you as the summer holidays approach."