Might sound a bit tame, but ...
For me it was the first flight, post-qualification, with a passenger. I.e. I'd set myself a goal of taking my wife flying, and, after checking out the aeroplane, and checking everything else I could think of, finally took off with her in the right seat.
Tracked east from Sherburn to the Humber Bridge, NE to Hornsea, up to Scarborough, W to Kirkbymoorside (orbiting over our house enroute) and south past Wombleton, Castle Howard, York and back to Sherburn via Selby.
Flight enjoyed greatly by both of us, and the sense of achievement was amazing.