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
Gen IV, solely because of HGSS. Gen II holds a special place in my heart. Gen I is were I started, but I found Gen II to be everything I ever wanted in a game. Until HGSS changed that by making it almost perfect.
Honorable mentions: I’ve really like gen XI because of all the quality of life changes, the story was surprisingly emotional (not great, but better than any other gens as far as I’m concerned), and the terastilization is cool as hell. It genuinly changes up how I plan my teams and I get to experiment with Pokemon I don’t usually like or that have types that ruin them. Gen VI was dope too. Megas were awesome and ORAS is the definitive way to experience gen III.
Edit: Also, Gen VIII because legends Arceus was refreshing as heck. Genuinly made meaningful changes for the better and felt good to play. Despite being a much more limited experience. It was focused and fun. Probably my second favorite Pokemon game. SHSW were pretty weak, all things considered, but Legends Arceus was special.
The first gen will anyways be best for me. Nothing else since had captured the magic I felt for that game with the exception of gen 2. Going through all of Johto and beating the game and then end up back in Kanto and beating that one too. That was truly special.
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.
I’ve lost track of gen numbers, I’m old, been playing since red/blue, but original gold/silver will always be the peak for me. Besides that I did especially love the rolling nostalgia wave of fan service in the Alola games, thought they were pretty fun but way too easy.
