Vaporeon, 1 I love watertypes and 2 Back when I got Firered I had chosen Bulbasaur as my starter. So somewhere along the line I neglect to get a water Pokemon, or anything worth using against my Rival's Charmander so he kept pwning me, but luckily I had Eevee and a waterstone and a couple useful water TMs, so I evolved it without thinking twice, gave it the moves and won, almost always got one in my team now.
And out of Legendaries Rayquaza, all because I found my first one on accident not even knowing it existed, since Sapphire I've had to keep one on my current game.