Paul
Gary
Ritchie
Jimmy
Some other League Trainers
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Ash

Satoshi-kun/Ash of course! but still I think he might already be a Pokemon Master because a Pokemon Master could be someone who understands pokemon and someone who cares for them and always tries to get better because no matter how you look at it everyone has room to get better even the champions of regions because they have to lose sometimes even same with gym leaders no matter how strong one is there's still room for improvement no matter how strong one's title says they are that probably is moral of it but anyhoo from that Satoshi/Ash,Shinji-kun and Hiroshi-san/Ritchie are my favorites of above.

Ash is a better trainer than the others in my opinion. Next would be either Paul or Gary but i think it would be Paul, since he's really strong and think he's a bit better than Gary.
And @MCHopnPop i so much agree with what you said! :)

He ties with Ash, but Paul has a better chance of actually becoming a Master.

By far! I mean come on he's the main character and they always have the main character win against the rivals.

Shinji!!! <3

KataraLover: Not always, it took Ash forever to even beat Paul and Gary. He still hasn't beat Trip yet.