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.
I assume you mean gen IX?
Yes, oops haha.