Ash's DP team kinda suck, since the only pokemon he used the most on his Sinnoh team was Infernape and Staraptor, the rest of his team was put in the back ground.
The OS gave the main characters much better development then the AG/DP series both did, Ash in the Indigo League lost and learn from his mistakes to become an even more better Pokemon trainer, the next time he see Gary he would beat him. The Johto League is were Ash gotten a lot better as a Pokemon trainer and finally beaten Gary his long time rival for good, and he did it all in the same series, he developed and improved for both regions instead of one. That right there is good character development.
First of all, did you even watch the DP anime fully? If so, when did you last watch it? Buizel, Gliscor and Torterra all got great development. I'll start with Torterra, back as a Turtwig it was a really speedy Pokemon, and Ash had relied on that speed and mobility as part of his strategy. After evolving, it grew, and so did it's weight. It was no longer as quick as before, and they both had to develop a completely new fighting style to compensate. Then came the Energy Ball power-up (I don't really remember how it worked) and Rock Climb. It gave it a bit of it's mobility back, and a rather powerful attack.
In fact, dare I say, I think I like Torterra a little more than Bulbasaur.
Buizel was originally caught by Dawn, and even when doing contests, he didn't really enjoy them. Aipom on the other hand, while on Ash's team, didn't really enjoy battles, but preferred contests. So, once Wallace noticed that, he suggested a trade, and that marks the first trade in the anime that actually made a difference later on. I don't know about you, but I loved that Lucario vs Buizel battle against Maylene. And I also feel like DP was the most creative out of any of the generations. With all the strategies that Ash developed. Like counter shield.
Gliscor was originally the weakest in it's group, it could barely fly properly. While Paul went and caught the leader, Ash decided to capture this goofball because he grew attached to it. While doing his best to train him, it took the group being in trouble and a cameo of Gary with a Razor Claw to learn how to fly, as well as evolve. Even after they evolved, they continued training, even leaving his party for a while to come back for the Sinnoh League knowing Giga Impact.
Infernape and Staraptor were originally Paul's and they were the ultimate decider to see if Ash's way of training was superior, or Paul's was. (Plus both of them are already overused in game, so of course he would use them a lot)
I don't have much to say about Gible though, besides Draco Meteor and it's habit of hitting Piplup, I can't remember much.
I actually liked Paul over Gary, sue me. He was just an all round better rival, putting a stop in Ash's stride, and imposing over him. I always put down that Elekid had perfect IVs, and the fact that they are both electric types is why they were originally a draw. There was development on all sides, pokemon and people alike.
You say that the Pokemon in the Original series had good development, but all Charizard has was that it finally listened to Ash. Not much development after that. :/ It's even sent away with Ash calling it whenever the going get's tough, Deus ex Charizard.
Butterfree was depressing, admittedly, but poor Primeape. As soon as he get's proper screentime, he is released. We never really see Muk. Pidgeot was just there for a bird. Bulbasaur and Squirtle, now..
Bulbasaur was always a leader, and it sort of stayed that way all the way through till now. It not wanting to evolve, along with Pikachu and Squirtle, while still being one of his go to Pokemon can't really be considered development otherwise. Either way, I love the episodes that it sees Ash again, it looks so happy.
Squirtle was originally part of a gang of released Pokemon that terrorised the city, and after all that happened, his gang became firefighter's and he joined Ash. He didn't get much development throughout, except for it's friendship with Bulbasaur. In comes Johto, the fire fighter's competition, where he sees that his gang has also grown. He rejoins them, and isn't seen until the Johto league where we find out all of his gang have evolved into Blastoise, so he is pretty much in retirement now.
We never really see Snorlax anymore either, too much to feed. And he has 29 Tauros most of us have never even seen. He used one in the Orange Islands, but that't it. They are also a running gag in running Ash over.
Ash's first capture in Johto, being Heracross was only there for a running gag of it wanting to eat Bulbasaur's nectar. He is strong, but also isn't used all that much. Used a little in the Johto league and Sinnoh league.
Next is Noctowl, his only shiny Pokemon. Again, used to promote shiny Pokemon, and Ash needs the regional bird. While used in the usual hunt team rocket and the few gym battles, it didn't really develop much either.
I'd say the only Pokemon in Johto with any extravagant development is Bayleef. Coming to terms with her own strength, and being careful while also showing her affection. Being jealous and the like.
Cyndaquil got a little, with being able to use it's full strength whenever he wanted without needing to warm up first. As well as an evolution in Sinnoh, which I feel was a little unneeded, but oh well.
Now it's Totodile, the Pokemon Ash fought Misty over. Not much to say about him, he just seemed like a playful child. Nothing comes to mind for him either.
Brock and Misty.. Brock did get some development throughout the series, but again, the most was in DP when he decided to become a doctor. He pretty much stayed the same throughout until then.
Misty on the other hand, was completely obnoxious all the way throughout the show. They were always arguing, and she never really accomplished anything. The only reason she followed Ash was because of her Bike, while Brock joined him to develop his own skills. (Still mad she stole Togepi BTW)
She is rude to her poor Psyduck, and she didn't even like Politoad at first. Considering her goal was to be a water type master, she didn't really do that much towards it. Brock was always learning. Her biggest development came in AG, where she released her newly evolved Togetic because she sort of had to.
Brock was always there as a sign of wisdom for Ash, and once he left, Ash was a little lost. (My head canon for why he was so dumb in BW. He didn't have Brock to help him) Although the running gag he had with every female they met got a little repetitive. Brock even got development away from Ash. When he had to win back his rock type gym from his evil step mother. He beat her with Ash like tactics, and won even against water types. In DP, he was in his element, always helping nurse joy, he even had a Happiny of his own. And Brock, while being a Rock Type gym leader actually has less rock types than his other Pokemon. That is development in my mind.
OK, personally my order of best generations are
1. XY (The best by far, writing, character, animation wise. In every way it is the best. Plus Amourshipping is canon, so even better)
2. DP(It is honestly really bloody good. Every character was memorable and human)
3. SM (If people actually sat down, ignored the new art style, and focus on what it is without comparing it to other generations, it is actually pretty good. It was never meant to be like the others, and Ash joined the school because it looked fun and he didn't have plans for another league. He wasn't forced into abandoning his dream)
4. AG (This is purely for Nostalgia, as I grew up with AG)
5. OG (Kanto to Johto, including Orange Islands)
6. BW (It was just a soft reboot, just like the games were meant to be)
Hate my opinion if you want, but it won't change how I feel. And I am someone who enjoys every generation, so no hate on the OG gen.
Almost forgot Krabby, that's how forgettable it was. :p So nothing really to say.
TL;DR DP has much better development and characters compared to the OG, but XY will always be superior. Sorry for the long post/reply. Just needed to get my point across