Warning: The following post is way too long, uses too many semi-colons (among other things), and is the opinion of someone that, along with his best friends, quoted Mewtwo for their senior-quote in high-school (i.e. Very Opinionated Pokémon Fan) .
Personally, I liked the Original Kanto series more, because it was new, Ash's only accomplishments were his Gym Badges, Misty and Brock still had pull (Pull=they mentioned it a few times when someone came to battle . . . like twice) as Gym-Leaders, and Ash's goal was set: The Pokémon League! Plus, I found it more entertaining. He was the best of friends with his Pokémon and his companions, and he got his butt handed to him sometimes by his opponent. But Ash got right back up, and used his mind (and Brock's and Misty's) to overcome his obstacles. It was three friends (not including the pokémon) traveling together, doing what they did, and liked best. Even the opening theme was amazing . . . and it fit so well, Ash ALWAYS said he was going to become the best Pokémon Master . . . later he did it sparingly, but in Kanto he ALWAYS said it, and the opening theme went perfect with that concept.
Then, I like Johto. It wasn't as entertaining to me as Kanto, but they tried a couple of new things that I genuinely felt worked well. The Narrator kept saying he was on his way to Johto (even though he was IN JOHTO) was really annoying. I felt mad when Ash left his Charizard; He trusted his Pokémon, he befriended his pokémon, he CARED for his pokémon . . . then he just left Charizard there because he accepted the fact the Charizard wasn't the strongest Charizard there was . . . of course he's not!, but you'd find a way to win with him anyway! It's almost like he lost his will after he made it to the Indigo League and he kept acting like he was just a Master going after another trophy.
I enjoyed the Orange Islands. I liked Tracey, but I think it would have been better if Tracey didn't temporarily replace Brock, so much as become a temporary fourth companion. I think Brock and Tracey would've worked well together. *Insert joke about Brock asking Tracey for pictures of Nurse Joys here*
Advanced Gen. lost it for me. At first I was accepting . . . "Okay . . . Misty isn't here, but she'll come back. Besides May is alright." And it was okay. . . but for some reason, Ash still acted like a rookie. I do have to admit, WHY follow Gary's example and leave all your pokémon. Ash repeatedly proved that he was better than Gary; Maybe not against Gary, but Ash performed better than Gary since day one. He won gym badges without having the rest that Gary had, he made it farther in the Indigo League, he won the Orange Islands Cup-a-ma-bob, and yet he still felt the need to leave ALL the Pokémon with Oak. I'm all for sending them to Oak after you catch a new one, but really, If Brock or Misty had kept accompanying Ash until he was already in Hoenn, he would've at least kept the ones that could've helped.