The Esssex located gateway, which rebranded itself as a London airport a few years ago, topped the poll with a customer score of 86% and a maximum five star rating in four of the seven categories assessed.
Passengers in particularly praised it for the short queues at bag drop, security, passport control and baggage reclaim.
It is also quick to point out that fellow Essex gateway, London Stansted, and Bedfordshire located London Luton finished joint bottom in the survey.
In summer 2015, 11,226 Which? members were asked to complete an online survey about their experiences of flying from a UK airport in the past year.
Categories surveyed included queues at Check-In, Bag Drop and Airport Security, Baggage Reclaim, Seating, Food Outlets and Passport Control.
Customer scores are based on a combination of overall satisfaction and how likely people are to recommend the airport to a friend.
Glyn Jones, CEO of Stobart Aviation that owns London Southend Airport, said “To be named ‘Best Airport in Britain’ for the third year running is wonderful news and once again testament to all the hard work that the team here at London Southend has put in.”
“Our unique ‘Simply Easier’ promises to passengers that include short walking distances, superb customer service, a lack of queues compared to other London airports, our own station and car parks right next to the terminal are all key factors in our success and why more and more passengers are repeatedly choosing London Southend Airport over all the others.”