Rattata because it is beast. It is in the top percentage of Rattata. The F.E.A.R. starategy would own Red's Pikachu.
But when someone uses Water Spout, Choice Scarf Blastoise they have to be a bad trainer. (Or use Blizzard on Snorlax or put Blast Burn, Hydro cannon, and Frenzy plant on the starters.) (How he become league champ, IDK he doped his Pokemon with Full Restores like Lance or something like that.) (And Red is separate from Ash, Ash is a disgrace to Pokemon, Red would own him.)
To all Ash haters: It says Red's Pikachu. Even though I am not a fan of Ash myself, I'm just saying, it's not the nicest thing in the world to pick on a 10-year-old.
AFter saying that, you'll be surprised to hear that my vote goes to Rattata.