I always thought the original and orange islands were pretty "meh." Sure, the original was what got me into it, but it didn't mean they were "that great." The reason behind my logic was because pity badges and below par fillers. I mean, not only did he not win the league (those episodes were awful IMO), he couldn't "catch em all."
I felt the same way about the orange islands. Never really saw why they did this, but it did make for and awesome movie (P2k). Tracey was such a boring character, and by that time, I thought of Misty as stale. The fillers were below par, especially since there weren't technically any new pokemon.
Johto was a bit better I suppose. They brought back Brock and dumped Tracey. Team rocket was still a joke, and all of their jokes started to really bug me. Misty pulling Brock's ear became really old really fast as well. The new pokemon/region was what made it better than original/orange. His gym badges were actually earned, and I thought the pokemon championship at the end of the series was fantastic.
Advanced was awesome. It was such a great refresh in every way. The new region looked promising, and the pokemon looked more appealing. It was also nice to see Ash in a different pair of clothes as well. Pokemon contests were such a huge hit with me. I thought May was an amazing refresh from where Misty left off, and it was good to see a change. Max was annoying when he was being a smart alic, which was about 50% of the time. And, this gen spawned the Deoxys and Jirachi movies, which were fantastic.
Diamond/pearl, my fav. It still latched onto Brock, which was fine by me. It was by far the best series up to that point. The writing was better, the fillers were better, and the animation was better. I was happy contests were still alive. Dawn was an okay character, as she was like the female version of Ash in many ways. The team galactic involvement was really well done. And Paul was the best rival Ash has had IMO, because he is the exact opposite of him. Unimportant, but D/P had some of the greatest opening theme songs as well. I know the original was appalling, but the three after that were dynamite. Aaannnddd... This gen gave us the darkrai movie.
BW was such crap, I don't even think I have to say anything, but I will anyway. The Ash reboot was really sad, he forgot how to catch a pokemon... Had some of the worst battles, some of the worst fillers, and IMO, fifth gen had the worst pokemon with a select few that I liked. Cilan was a step down from Brock, and his cooking puns became old fast. Iris was an annoying character that had so much potential. Her "you're such a kid" annoyed me so much after she would turn around and act like one herself. Team plasma started getting involved way too late. The only positive was team rocket's development, albeit a bit late to the game, but accepted nonetheless.
Having watched the first 11 episodes of XY (2 english, 9 japanese w/subs) this looks extremely promising. I guess that's writers for ya. Clemont and Bonnie are really likeable characters so far, and Serena seems to add a different dimension to the show that we haven't had in ages. Ash seems to show more signs of maturity, and hopefully he warms up to the idea of Serena's feelings for him. I think the few fillers we've had so far a great because they show some sense of character development. And my gawd the new animations!
Sorry for the long post, but this was a fairly easy question for me to answer, and I had the free time to sit down. My favourite was definitely D/P. I really couldn't let nostalgia take over practicality and reason. D/P for its time had the best writing, animation, and character cohesion. I've got my hopes up for XY, and we'll see where it goes from here, with XY12 airing in japan next week. In the writers we trust.