Okay dude....Cool pokemon depends on the person. And that's not Ash getting rid of Pokemon but him growing as a trainer. And as trainer --- trade, redirected training, or setting free, is part of the whole deal.
First of all Charizard leaving his original team was the bes thing ever. Because at the time Ash had become to rely on Charizard's power too much and his Charizard wasn't anything compared to ones met at the Charicific Valley.
Primeape was the for same reason. Redirected Training. It's still Ash's pokemon but the time of having him, he couldn't control him. He was given to a special Pokemon P1 trainer and that was for the better. (Specifically if you watched Pocket Monsters and NOT Pokemon....a special in the JP version where all of Ash's pokemon shows up)
Pidgeot was set free because it gained a home in the wild and Ash couldn't stand it being seperated from it's new friends. Although THAT was his most retarded hand off....It's pokemon. Find one episode where anything is so harsh and selfish (other than Satoshi's beatdown to Shinji about 10 episodes ago in Pocket Monsters)
Butterfree was a set free (UNOFFICIALLY...since the signature on Butterfree's pokeball is still active) after IT found a mate. Ash HAD to part with it.
Haunter NEVER belonged to Ash other than it's use for one gym.
Ambipom, I have to say is the ONLY one I'm sad that isn't going to apart of the group. Because it's purpose for leaving is stupid. What was the point of building up all of that Battle Experience and all of that Coordination experience only to have it go off and hit fuxing balls between another pokemon and human. THAT IS JUST LIKE....I honestly don't have words for it. But I guess that says something about Ambipom. For it to just jump up and say "I'LL CHOOSE PING". Basically it was the cast's most ADD Pokemon other than Psyduck who really just had a short attention span.
Only charizard shows up in Battle Frontier dude.
And guys...seriously? I do understand most of you are probably less than 15. But here's an idea. Watch the shows in Japanese. Pocket Monsters is current at DP146 (612 episodes overall) where we get to see James (i.e. Kojiro) dress up like Jessalinda. What's better than seeing that about 40 friggin episodes early.