My nostalgia goggles tinting things a bit here, but Gen 2 (GSC) felt like such a jump in quality compared to RBY; especially Crystal.
Moved to full color
Animated sprites
Character selection
RTC & the day-night cycle
Both regions & 16 badges
QOL mechanics (hold items, eggs/breeding, A to use HM, rematches, Mom bank, Move Deleter)
Dark & Steel type additions
Johto gets a lot of flak for the level curve, but the game just felt so big at the time, seems like that should count for something. Also I just like its vibe.
I don't think any other gens have felt like such a complete step up compared to previous ones for me. B&W came closest, probably, was bummed they didn't keep seasons. Made the world feel more dynamic. The 3D stuff is fine, just feels more wooden. They lost a lot of the charm moving away from pixel art.