Perhaps not my fondest, but as of recent, one of my most memorable would be when I was racking up a high score in an old Disney skating game with a Sabaton album playing in the background.
I wanted to see how far the score counter would go. Would it end on nine digits? Ten, eleven? I stayed up for hours trying to reach that score and see how high it would go. Didn't stop on nine, didn't stop on ten, carried through to eleven. So I kept going. I was determined to see exactly how far this counter could go.
All in all, must've kept at it for about four to five hours, with a short break here and there. I know I stayed up for at least five hours so you'd need to subtract the time of my breaks to get an accurate answer. While I'm not exactly sure how long my breaks were in total, I know for sure it was less that an hour.
At one point, I almost gave up. I believe this was when I hit eleven digits and began to realise it wasn't going to stop. But I somehow found the motivation to continue, and so I did, going on until I reached twelve digits. This was near the end of my run and my thumbs were beginning to hurt noticeably.
Shortly after I hit twelve digits, of course, I finally stopped and went to sleep. I took a photo, timestamped it, and left. I've since lost the photo but I know my score was rounded to 125,000,000,000 by the nearest tenth digit place.
In short, it was perhaps my greatest gaming feat, and I'm proud of it. It really shows what lengths I will go to simply to prove a certain task can be accomplished. Or test the limits of a game.