My first Lv.100 was my Furret in Gold named Choclate. I was in Mt.Mortar and stayed in there for like twenty hours, and now Choclate's havin' fun beating my friends and whooping Stadium 2 Red's Tauros into next week.
Although not considered to be mine, i helped my brother in completing for more than half of the pokedex in Sapphire. Beating up E4 countless times for the sake of giving exp to the party for evolutions with exp share. i can't really remember whether it was kyogre or rayquaza who maxed out its lv first.