Well, I'm definitely going to go with season 1 for several reasons. Such as how well the original character trio came together and enjoyed much interaction and daring adventures. In my opinion, the key to a good story is good characters and they certainly pulled it off well.
Another reason is how well they balanced action and comedy. Throughout the season, the battles usually aren't too focused on, yet there are plenty of other chaotic events that are as humorous as they are exciting. This also ties in with the characters in a sense that they intertwined the humor into the characters personalities in addition to giving them great lines. I suppose that having been given good characters and amazing dialogue is what compels me to support the original voice cast.
But possibly the best reason is the virtues in the anime back then. There was always a huge focus on friendship, following dreams and adventures, helping one another in a difficult situation, and caring for other Pokemon and people, exploring and appreciating the world together. Not to say that's entirely gone today, but it still feels greatly overshadowed by the battling, rivalries, stereotypical villians, etc. I don't think TPC knows hardly at all what they are doing with their characters or dialogue either. The times have changed; Pokemon doesn't feel like it has much of a soul anymore. It doesn't feel passionately crafted to carve its own niche in the television industry today. I'm not going to call it horrible, but I feel that it should be something more and someday live up to the standards set in earlier seasons.
I am the current leader of Save Our Voice Actors, a Pokemon fan group supporting the voice actors in the anime dubbed by 4Kids (seasons 1-8). Our goal is to provide The Pokemon Company international with encouraging feedback in support of returning the main cast members. If you're interested, please visit us at facebook.com/saveourvoiceactors