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.
3 was my first, 5 was the first (and only) one I legitimately own
I have fond memories of both, though 5 edges 3 out by a bit