So far, it would be completing my five hour flight for the Silver C (don't have the badge yet; still haven't done the 50 km distance flight (shameful)).
For a number of reasons, I didn't launch until about 2 pm., and after I'd been up for about three hours some cirrus moved in and began shutting down the thermals. Everyone else was shot down and they all assumed that I would never manage to stay up.
After four hours, I was at 1,400' over the field. "I've had it". I thought. But I still had 500' to play with, so I flew a search pattern, located a weak thermal, and (slowly) climbed back to 4,000'. I played that out for about 30 minutes, then climbed back to about 2,200'.
At the five hour mark I was down to 1300' again, but I couldn't land yet so I flew around at 42 knots in zero sink and managed to stretch it out for another twenty minutes. I had been somewhat tired and a bit sloppy after the first couple of hours, but now I really worked at doing the most precise flying I have ever done: that yaw string was straight back all the time!
It was a great feeling of accomplishment, and almost four years later, I still remember all of the details.