Beating Lance in Silver. I was 9 at the time, not knowing what to do, and I battled Lance. His last Pokemon nearly swept through my whole time, but I had a level 30 Poliwhirl that managed to avoid it's first attack, before fainting it with Hydro Pump - it was low on HP, so it didn't matter how weak the attack was, just as long as it's HP dropped to 0.
Really, I nearly cried when I won.