For me a close thing between the Vulcan "howl" and the sound of ten Merlins directly overhead when the Canadian Lancaster visited. I'm lucky enough to live on the route out of Southport Airshow
Stood in the midst of fields as 2 Lancasters, a Hurricane and a Spitfire pass directly over your head at around 500ft is just an amazing experience. (The BBMF Lancaster pilot used to live in the next village so makes a point of flying out that way after the show).
Other than that, a squad of 4 F111s going up Loch Ness at close to zero feet competes with a B1-B doing a touch and go with all four afterburners at Woodford airshow as the loudest sound I have ever heard in my life, and I've played in brass bands for most of it......