A powerful trainer has entered your domain. He has defeated all of the trainers who guard the pathway to your throne. A battle has ensued, and you are down to your last pokemon. Your last pokemon is the definition of your character, and soul of being a trainer. It is your favorite, who has been with you ever since you met, and has always been the most powerful reserve in your arsenal. You throw the pokeball...
GO RAYQUAZA GO! *Rayquaza looks confused, then rids the town of all life*
In all seriousness, Arcanine, Squirtle, or Raichu would come out. A tough pick, but I would go with Raichu. I'm a bit partial to electrics, and Raichu is my favorite. I don't know what a Raichu's personality would be like, but looking at one, I would say lazy, tough, and easy to make angry but otherwise a very nice pokemon.
Mewtwo. Cause he is named after me. And he is my strongest Pokemon. Level 100. Knows Physic and Shadow Ball so he can take out pretty much anything that could hurt him.
Salamence and Garchomp were classified in the Uber range for competetive battling, but they're not exactly true legendaries... more like psuedolegendaries