This is a tough one, defiinitely...
On one hand, Charizard is a massive powerhouse behind Ash's team and had a very dynamic personality, going from a codependent little lizard to an arrogant, fear-mongering dinosaur, and finally to the dragon anti-hero turned close friend that we all know and love. I believe everyone has a place in their heart for Charizard. He went through his rebellious phases to try and prove his independence and eventually realized that everyone around him only wanted to be his friend and care for him, and he finally decided that he's better as part of a team than he ever was by himself. Charizard is definitely one of Ash's greatest allies and friends, and he could easily win this poll.
However, let us now turn to Bayleef. When Ash caught her as a Chikorita, she displayed a massive amount of will and determination, as well as battle prowess. She always listened unfailingly to Ash's every word, and she consistently attempted to prove herself to him. Also, certain evidence can suggest that she actually developed a sort of crush on her master/friend, and went out of her way to help him and show him affection. When Ash scolded her in "Turning over a New Bayleef!", she was understandably upset, as someone she loved was mad at her for something she did not intend to do. Plus, Bayleef was the only one of Ash's Pokémon to evolve during his entire travels throughout Johto, doing so in "Current Events!" (Phanpy and Cyndaquil did so as well, but this wasn't until MUCH later). As such, it can be seen that Bayleef was also a very important member to Ash's team, and perhaps held an even greater place in some people's hearts than Charizard, who has now been turned into somewhat of a cliché.
So, I guess my vote will go to Bayleef. Here's lookin' at you, kid.