Nothing quite as fast and powerful as some have described, but I've got two that stick in the memory.
The first was that unforgettable 'Big Wing' at the Duxford Battle of Britain display in September 2000. 19 Spitfires, 4 Hurricanes, and thousands of people with lumps in their throats.
The second (or first, if we're being chronological), was around 1989 or 90, while I was flying a C150 out of Southend. Overhead Osea Island on the way back in from a local area bimble, I heard a callsign contacting Southend approach and saying they were also overhead Osea Island. Obviously a bit worried about getting run over (!) I started scanning the sky just that little bit more carefully, craned my neck forward to peer up from under the front of the Cessna's wing... to see the BBMF overtake me, 1000 or so feet above and about three quarters of a mile over to my right. And that's the closest I'm ever going to get to flying in formation with them! <img src="rolleyes.gif" border="0">