For me every flight is still wonderful but for special mention:
First Solo (of course)
Landing on Lundy (always wanted to do this since first reading that it was possible in Pilot magazine.
Two parts of the IR flight test:
(i) The examiner taking down the screens and seeing that I was actually over the approach lights at EGHH after a gruelling 9nm inbound tracking for the NDB approach (reason: I had previously failed the F170A twice on the NDB approach and it was the end of a very long day. The test had started at 0900 and we landed in the dark!)
(ii) The examiner telling me I had passed as we taxied back.
There are so many, but you asked for only one. Sorry I couldn't limit it to one.
By the way Whirly, you don't seem to be on the Sherburn list but you started the fly-in thread. Are you coming this Saturday?