This is a difficult question. Let's see:-
First Brock. In the beginning episodes, he seemed like a force to be reckoned with, and someone who can't be beaten by any more than an avergae trainer. However in the next few episodes, it was evident about his problem .Anyway he has come a long way, and has been a dependant guy. He knows a lot about poke'mon and computers(as shown in the Bat episodes if you what I mean).
Next Misty was pretty good. She was just a water type trainer who went on to become a wate type gym leader. In the chronicles, it was shown how she passed the tests set for her, and how she became a true gym leader. As for character, she could be irritating sometimes, and her fear for bug also is a minor point(even if it is meant to be a comic relief).
Next Tracey. In terms of achievement, he is at the top. You might be asking why, but from an ordinary(but talented) poke'mon watcher he became the assistant of one of the most famous and most knowlegeable person in the poke'mon universe, and that is a real big deal. However due to Tracey's limited screen time, his character development is not too much.
For May, she was just a girl who didn't even like poke'mon but turned out to become a confident co-ordinator defeating a person like Drew, and ending in top 4 of a GF.
Max was also pain in the neck(no offense). He knew about poke'mon and loved them from the beggining, but hasn't shown much character development apart from dragging Brock away from Nurse Joys and Officer Jennys. However he has shown great potential when he saved Raltz and evolved Mighteyana etc.
As for Dawn, she used to lose every time. However there was always the pressure of living upto the family name. But she soon got over that, and managed to beat even May(I can't figure how) to win the Wallace Cup.