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Red & Blue:
Hard to really place objectively. It's fun to see the franchises roots and play with all of the funky glitches. Definitely enjoyable to play at the least.
Gold & Silver:
Fucking comfy, easily superior to Gen 1, most ambitious games to date.
Ruby & Sapphire:
Don't hold up super well, kind of underwhelming, I'd give them a pass
FireRed & LeafGreen:
For me personally, the beginning of the golden age. These games blow the originals out of the water and add so much to do and enjoy. I have a lot of good memories of completing the kanto pokedex on these bad boys
Emerald:
Absolutely amazing, completely render RS obsolete. First great postgame in the franchise.
Diamond & Pearl:
Suffer from the same issues as Ruby and Sapphire, starts to show that first versions aren't very good. Not the worst games, but not really worth your time
Platinum:
Emerald 2 electric boogaloo. Fixes a lot of issues with Diamond and Pearl on almost every level. Truly good game.
HeartGold and SoulSilver:
Best mainline titles in my personal opinion. Absolutely remarkable. You can easily get 50 hours in even on multiple playthroughs. The peak of the franchise in many ways.
Black & White:
Competent games, but my least favorite games so far. Should be held in high esteem but I personally don't like them.
Black 2 & White 2:
Definitely improve on the previous games, but suffer from some strange choices.
X & Y:
Admittedly I hated these games on my first playthrough, they kind of grew on me over time though. Play them if you're bored.
Omega Ruby & Alpha Sapphire:
Real dogshit, and heavy disappointments coming off of the trend set by FRLG and HGSS for remakes. I liked the soaring mechanics and that's about it.
Sun & Moon:
I genuinely remember nothing about these games, really disappointing
Ultra Sun & Ultra Moon:
Same issue as previous, Necrozma was cool though
All told: The franchise peaked a decade ago and it's a shambling corpse at this point.