For me it’s definitely gen 4 (heartgold and platinum are just. top tier), but gen 5 is really close. Really polished and challenging games all around
I also think gen 7 is pretty underrated, especially USUM which got a bit of hate for being too similar to Sun and Moon. Which is kinda true…but I think the end result was pretty good all things considered.
Gen 5, pretty easily. It was the only time theyve ever tried to make a new game instead of iterating on old ideas. The story is about as good as you can get for a pokemon game too