https://youtu.be/IWr6U4Ava68
This video shows all ashes Pokemon from gen 1-7. Bulbasaur and Charizard seem to be the two most used, bulbsaur was used a bit more through Kanto and the orange islands, bulbasaur was also used a bit more during jhoto.
The hoenn Kanto battles(forgot what it's called 0w0) Charizard was used slightly more than bulbasaur but not enough nearly enough to make up for jhoto and Kanto.
Non of them were used until unova where Charizard appeared on a island after ash completed the unova league, but Charizard is only there for a little while tho so I'm not sure but it's either bulbasaur or Charizard I kinda think it's more bulbasaur but could be wrong :D