Technically, I raised a Mew or a Raichu to lvl 100 first in Blue, but since that was through exploiting the Missingno. Rare Candy Cheat, I'll just say it was my Gardevoir, Sana, named so because of the romanji of Gardevoir's Japanese name, Sirknight (Sanaito), that I raised to lvl 100 first. I started raising her in LeafGreen to lvl 74 and then I sent her to Pearl when I got to the Pal Park and finished her leveling there. My new Sana in Platinum, from an egg made by the first Sana and my shiny Gallade, is following in her footsteps. She's currently lvl 47.