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.
Subjectively, I love Hoenn. It has that feeling of being at home when I'm there (especially the originals).
Otherwise, I loved HGSS, they really made the originals shine.
I was bored playing SUMO. Had to stop for a year before taking it back again, like XY. Not enough exploration. That being said, I miss this region. Alola had a slightly different feeling than all the other region. Something chill, made me feel being on vacation, ahah.
SWSH was pretty bland. I managed to finish it without dropping it but I never bother returning to it after the story ended. In the end, I sold it. I never sold a Pokemon game before, knowing that I have each generation physically.
Legends Arceus was great. Weak technically, but it was so fresh and fun. That's the first time since a long time that I want to complete the Dex.
I'm waiting for the price of Violet to drop before giving it a shot.
EDIT : I didn't mention BW and BW2. I really loved the idea of not having pre-gen 5 Pokemons until endgame. It helped to have a better feeling of discovery and freshness.