Can't limit it to just one, sorry about this:
First solo - I was having a ball, not at all bothered until I turned base, saw the runway, and thought sh1t, they want me to land this!
QXC - still a great highlight. Little old me, flying along the edge of Birmingham's airspace getting a flight info service from them (don't believe a word you hear - those guys are great) and a good luck message. How did they know?
First spin - wow! Got to do aerobatics one day soon.
First XC following my PPL. Flight to Caernarvon and overhead Bangor I was treated to the sight of Anthony Hodgson's Spitfire doing an impromptu display close by, and then beating me to the airfield by an indecent margin.
Recently on a beautiful day climbing to 6,000' above the Welsh hills (close to you Whirly) seeing the shadow of my C172 on a nearby cloud and a circular rainbow closeby. A passenger tried to take a photo but it didn't come out
Now if anyone starts a thread on really bad moments, I've got a couple of those too.