Which Starters are Canon (Kanto)?

According to Bulbapedia, in Pokémon Red, Green, & Blue, if the player were to leave Pallet Town without taking a starter Pokémon from Professor Oak's Lab (which is impossible during normal gameplay for reasons that should be painfully obvious to any sensible person), the game, instead of breaking down, acts as if the player had taken a Bulbasaur. The rival would, therefore, have a Charmander as their starter.
Furthermore, as mentioned by @BlackButterfree, Ken Sugimori, the art director and primary character designer for the Pokémon franchise, drew a picture of Red, Blue (known as Green in Japan), and an early version of Leaf facing off against each other, with Red commanding a Bulbasaur, Blue owning a Charmander, and Leaf having a Squirtle.
With all of this in mind, I'd consider Bulbasaur to be the "default" starter for Red. From there, I'd conclude that Blue would pick a Charmander just to spite Red, while the Squirtle would go to either Professor Oak or Leaf, depending on whether or not Leaf exists (if she does, Squirtle leaves (heh) with Leaf; otherwise, Squirtle remains with Professor Oak).