Beating Brock in Pokemon Yellow. It was my first pokémon game so I was a little stupid and didn't knew type matchups that well (except for those told in the show) so I had no idea Mankey would rape Brock so I beat him using only Pikachu and Pidgey, Pikachu was level 20 and Pidgey 17. Pidgey ended up the fight by killing Onix, gained a level and evolved after the match.
Felt awesome.
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Best LPer ever, Gix ! : http://www.youtube.com/watch?v=Vt5YKmP7ecY