That'll probably have to go to his Charizard.
Ever since he was a Charmander he was nothing but trouble, constantly picking fights, disobeying Trainers, and snapping at everything that annoyed him. Basically imagine Bakugo from MHA

Tyler saw potential in Charmander, and after rescuing him from Team Rocket, he gained enough trust for Charmander to be caught, but he was still as grumpy as ever

Charmander would listen to Tyler as it grew up into a Charizard, but it was still really rebellious, it still is honestly. But Tyler and him have really bonded over the years, even if he doesn't show it

The best example of this was when Tyler mastered Mega Evolution with Charizard