Ok, here's my ranking:
1) Advance Generation
I find this series the most balanced of the bunch. It has Ash as a quite competent trainer and with a bit of personality (he was shown as cocky since he acknowledged his past achievements and I liked that), May is my favorite girl since she had great dynamics with her brother Max and introduced Contests and Brock had still a good role in the group. I loved the League and the pacing was good. Also, it has one of the most epic matches ever (at least to me): Charizard vs. Articuno.
2) Diamond and Pearl
I simply loved how Ash was shown as a competent trainer. He seemed to really know what he was doing. I really enjoyed his rivalry with Paul and the battles were excellent and full of strategies. What I didn't like was that Brock seemed almost non-existent and I wasn't really fond with Dawn and Piplup at the beginning of the series.
I began with the beginning, so I'm quite fond with that series. It was quite humorous and had a lot of interesting stories. Also, the characters had a lot of... character. XD The battles weren't great, but I don't complain. They had just started after all, so I forgive them for that.
4) Orange Island
I liked the originality of the series, since it wasn't based on any game and I liked the original Gym matches and the champion Drake. But other than that, I don't remember much about this series. It simply didn't stick much for me.
5) Best Wishes
I like the animation and the personalities of the characters. However, I'm not too fond to the rest. I find the series quite messy and too fast-paced, in all honesty.
You may say that kids follow the show because we can see monsters fighting. Well, I was a kid back then and I grew bored fast with a lot of episodes. Also the fights weren't that great, even if they significantly improved from Kanto (the League was really good, though).