My dear Pidgeot Yellow, from Pokemon Yellow. Caught her (technically "it" back then, but I always considered her a girl) as a Pidgeotto from Viridian Forest and she went on to beat Brock's Onyx...somehow.
From there, she, along with a Vileplume and Pikachu, carried me through the entire game. She was level 87 when I took the Elite 4 on for the very first time in my life and took most of them down.
I wish I'd had the sense to preserve her through the ages and somehow bring her forward to the current generation. I was happy when she reached level 100 in Mewtwo's Cave, maybe her incarnation that I've been raising in this game will help me tame Mewtwo once again.