I watched Pokemon when I was a kid, but like a lot of people I grew out of it around Hoenn/Battle Frontier and hated Diamond and Pearl when it first came up. But years later, I forgot what caused me to, but I decided to marathon the entire series from start to finish and.......... I like it.
Kanto was a fun nostalgic romp filled with early installment weirdness, it had mostly fun episodes, fantastic music, and was for the most part well paced. Negative points would be some episodes don't hold up quite as well, but other than that I didn't find anything to bad
Orange Islands was the same deal, it was short enough that I felt it wasn't dragging, the Gym Leader battles were inventive and original, and hey Ash actually won a League for once! A shame it's filler.
Johto was a bloated padded out mess with tons of pointless filler (hello Whirl Cup arc), there were some decent episodes here and there but aside from the Pokemon League and the Gym Battles it wasn't worth sitting through. Still though it was great to see Ash vs Gary as well as Blazekien in all of his magnificent glory.
Hoenn was great. Aside from being well paced it had a good advancement on Ash's character as well as giving the newbie character May much more development than Misty as well as having a consistent goal by competing in Pokemon Contest, which was a nice alternative to battles every now and again. The Pokemon League was eh though.
Battle Frontier was good, as well as nostalgic. I enjoyed how Ash rotated his roster using the various Pokemon he caught over the "years". The Frontier Brain battles were also pretty fun.
I hated Diamond and Pearl when I saw it years ago, now? I quite like it. Though it has the same pacing problems as Johto, it had way more of an overarching story to redeem it somewhat. Say what you want, but I thought Paul was a great rival for Ash and it really kept D&P interesting. D&P in general put a lot of emphasis on battle strategies and planning that really tested Ash's skill as a trainer and put him to the limit.
And finally, Best Wishes. It improved in some aspects, but regressed in others. Most notably Team Rocket for improvements and Ash's skills for regression.
I would like to explain more, but those are my brief thoughts on each series. I still quite like it, it's nowhere near perfect, but it's still enjoyable.