Gen 4 is probably my favourite of the bunch. Sinnoh is my preferred region geographically, I really like the story, and the music is great. I also have a lot of nice memories of playing Pearl as a young teen, when I still had the heart of a child, and having a blast. Then, the year SS came out was the first and last time I ever looked forward to springing forward and losing an hour (it brought us closer to the release day, after all). After HGSS came out, the stigma of liking Pokemon seemed to vanish and many people who had grown of the series came back. I have a lot of positive associations with Gen 4 and remember it very fondly.
Honourable mention to Gen 2 and Gen 5. The former was my first and the latter was quality all around. Gen 7 is too early to call, but I loved the more plot-focused story and seeing the characters be more involved in the going-ons. Plus, Lillie and Gladion are pwn. And Alola Vulpix.
As for least favourite... I'd have to say 6. Although Gen 1 does have notable flaws, most of which have already been mentioned: bugs, type inbalances, Wrap, graphical quality, ect. However, it was gen that started it all, and the programmers did have to work with limited tech. And, at the time, it was legendary, which keeps me from calling it the worst outright.
Gen 6 did bring some great Pokemon (ESPURR!), Pokemon Amie, and character customization to the table. However, with that being said, the story felt very weak up until the climax and Team Flare was a joke. Having a 5 man band of rivals could have made for unique and interesting character dynamics, but that did not happen. Instead, the rivals all felt rather one note and didn't develop too much, unlike in the preceding and proceeding gens. I should also note the gym leaders; other then Korrina (and maybe Valerie and Wulfric) they were rather forgettable and had no major role in the story.These flaws are feel even more striking, as Gen 5 was so strong story and character wise. I just feel Kalos had A LOT of wasted potential.
I don't hate Gen 6, don't get me wrong, I had a lot of fun playing through it the first time and adored my team. Plus, as I mentioned, it brought a lot of great things to the table! It just feels like it could have been much better then it turned out being.