It's not a competition in the strictest sense, but my proudest win was at a high school charity day, one of those things where classes run stalls to make a bit of cash. I was in the first year so 11ish, and at the time absolutely loved Streetfighter II, which I may have had on the SNES at that point. Anyway, one class of older lads had a console and was taking a quid or whatever for a go, I didn't have any money so was just happily watching the fights. One guy who was a game down (playing as Chun Li) suddenly had to leave so looked around and passed me the controller, suddenly there I am, a tiny year 7 playing against a year 11 lad already 1-0 up, with all his mates watching on.
I was never even a massive Chun Li player (always just played as Ryu) but I did know that she could jump off the back of the playing area and I knew how to do her special moves.... and so I somehow managed two victories over this big year 11 lad who had to sit there and take the mockery of his mates. Sadly I didn't have a pound to carry on playing.