Well, acually that's a hard decision. I'll name the top three I would choose!!!!!!!! If I had a decision, than I would choose Turtwig. I would choose him because Torterra can beat both Infernape and Empoleon. Also, Turtwig is pretty cute.
If I couldn't choose Turtwig, then I would choose.......let's see....hmmmm...Treeko!!! I would choose Skeptile for it's variety of moves and and high speed.
If I couldn't choose both of them then I would choose (not really since you have to use him in Pokemon Yellow) Pikachu. I would choose him for his high speed and power. Also because it's cute and Ash's pokemon.