This feels like it should be for the General discussion than just for anime, because even in the games, NPCs have mentioned that Pokémon are smart. That's why the number of badges determine how long Pokémon (traded Pokémon, though I think Pokémon you caught on your own do this too) can obey you on command. But I can see why you have it under the anime discussion forums, since we see this more in the anime than in the games.
Speaking of disobedience, the biggest one we can all think of is possibly Ash's Charizard. Now while Charmander was willing to listen to Ash, once he evolved into Charmeleon, he began to refuse. Now why is that? Is it because Ash is inexperienced, or was that just a Charmeleon's nature at first? There were some battles Charmeleon refused to do, like with Paras. Could it be because Charmeleon knew the Paras was too weak for it to have a fair fight? That's what's mentioned in Charizard's PokéDex entry. I think Charmeleon grew more rebellious after the fight with Paras because Ash kept pushing and pushing him to fight the poor little thing. He got even more rebellious as a Charizard.
Then it took about a season's worth before Charizard would actually listen to Ash out of respect. After seeing that despite all he did to his trainer, Ash still cared for him and stayed by his side to heal him. Thus, Charizard became a better Pokémon, as did Ash as a trainer by proving his worth.
Meowth possibly proves that Pokémon are smart enough to imitate humans, but most of them don't seem to think about it. They probably don't even care to in many cases. Meowth only became like a human out of desperation to win Meowzie's heart, even though it made him a freak among his kind. It's also hindered his ability to evolve, at least I think him learning to walk on two feet did that, but I don't think it was ever explained well enough.
Other Pokémon have also proven to be able to speak as well, but most of the time they're very powerful Pokémon, or Psychics--which are believed to be among the most intelligent of Pokémon. But it probably just depends on their genetics--maybe. Because if that was the case, if Mewtwo is able to speak through telepathy, then Mew would be ale to do so as well. But we never once seen a Mew use telepathy. Does that make Mew less intelligent than Mewtwo? No, Mew probably either hasn't learned that technique, or just decided not to. Mew showed little concern for the humans outside of saving Ash anyway, so there was no need to speak to everyone.
Now Dragonite and Latios are believed to be smart enough to understand human speech, but it doesn't mean they can speak it. So it most likely just depends on the individual Pokémon about whether they want to speak or not. Just like a Pokémon can decide to evolve or not, like with Bulbasaur and Pikachu.
So it's all up in the air about Pokémon being capable of learning human speech like Meowth, but we can all agree that just about all Pokémon (whether they act like it or not) are intelligent creatures able to think for themselves under a given environment. In which case, a Pokémon being with humans are possibly more intelligent than a regular wild Pokémon of the same species due to where they live. Have you ever seen another wild Mr. Mime take up house-cleaning?
...bad example, that was never fully explained why that Mr. Mime just showed up and took up a broom. But obviously Delia is considered that Mr. Mime's trainer, so I guess it counts...
