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.
Mechanics wise, gen 4. The physical special split was one of the best things to ever happen to pokemon.
Games wise i’d say gen 5 unova is such a cool region and I love all of the pokemon there, although new pokemon wise I prefer gen 3