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.
As incredibly flawed as Scarlet and Violet are, they make Gen 9 really compelling for me. Love the new Pokemon, playing through with a friend in multiplayer was a blast and being able to have my Pokemon walk with me is great. It’s an unfinished and laggy game but I haven’t had so much fun in Pokemon since Gen 5. Completely get why not everybody likes it, but I’ve had so much enjoyment from it.
As for Gen 7? I agree. USUM’s only major flaws are linearity (which I would argue was mainly bad for replays but fine overall), and that it removed what made SUMO so nice on its story. Ultra Beasts remain my favorite idea the series has done and the games had all Gen 1-7 mons plus megas plus Z Moves and the music was amazing and Battle Royale is my favorite multiplayer mode. Massively underrated, will be popular in three years when everybody suddenly loves it because old thing good, new thing bad. Mimikyu’s amazing.
SV is interesting to me because, i feel like at times it’s the best pokemon game (story, music) but at other tims it’s the worst (glitches, lag). I personally have it decently high in my rankings, but it’s weird how inconsistent it is lol