I'm quite surprised to see many people list my least favourite Gen as their favourite. I suppose it comes down to what you want out of the Games.
1) Gen 7 - The change in structure to the game worked well, and whilst I can't be doing with the Z move animations, I believe many of the new Pokemon work extremely well and give loads of different options to experiment with in terms of battle strategy. The world and variety of New Pokemon are both well suited to each other.
2) Gen 4 - This is influenced by this is where I really started to understand the battling mechanics, which made the games so much more interesting for me. Adding to that the new hardware gave the developer greater scope for environmental variety. I'm still hoping for a remake on the Switch. Admittedly though, the slow pace of the battles is very grating when I try to revisit them.
3) Gen 2 - Builds on Gen 1. Pretty much the same as the entry below.
4) Gen 1 - I game I cant help but look through with nostalgia glasses. Even 20 years on, the game still holds up for me, shown by the virtual console release. Yes, its basic by modern standards, and the Lets Go remake highlight how shallow it is in terms of features, but I think the classic version suits it better. There is more of a sense of adventure in it from this angle.
5) Gen 6 - Another one I really enjoyed at the time, but was completely shown up by Sun and Moon. Early on in the game it really seemed to lack and identity, with a fairly bland feel. This did disappear gradually, but it did feel like the developer was just finding their feet on the new hardware.
6) Gen 3 - An entry I enjoyed very much back in the day, but when weighed up against other entries is just feels shallow by comparison. There's nothing I really hate about it, just a lack of things that push it on.
7) Gen 5 - This Gen brought next to nothing new to the series. Its main selling points are the revamped graphics to better utilise 3D, and the strongest storyline in the series. However, neither are really the main draws of the series. It also has some of the worst Pokemon designs ever, featuring the embryo Pokemon, afro Pokemon and ice cream Pokemon. Also many look like direct stand-ins for Gen 1 Pokemon, so it all just stank of lack of ideas and/or creativity.