Justify your opinions and have fun discussing!

Gameplay (engaging challenge, Pokemon availability, balance, etc.)
BW2 > Platinum > Emerald > BW > RBGY > HGSS > FRLG > Crystal > XY > ORAS
BW2 has the best mix of a large dex, chellenge mode and varied Pokemon available to use. ORAS and XY's handholding and awful balance with the xp share is pretty inexcusable.

Music:
ORAS > BW2 > HGSS > BW > RSE > GSC > RBGY > DPPt > XY > FRLG
Combining the capability of new hardware with 'Pokemon' music in mind was (mostly) a good idea and I genuinely think ORAS has a fucking god-tier soundtrack, while XY's is almost the worst because a lot of the music doesn't have a 'feel' to them, they're just nice sounding instruments. I think FRLG remixes of the old music went in all the wrong directions.


Atmosphere (how good the region was, general tone of the game, writing, story, etc.)
BW2 > HGSS > GSC > RSE > ORAS > Platinum > BW > FRLG (Sevii islands) > RBGY > FRLG (main game) > XY

BW2 builds on BW in the best way by bringing you into Unova after time has passed and seeing how it's grown from there. But you don't have to care, since you're another person, but you can feel the impact of someone's past actions and enjoy the story either as someone in the know or not. HGSS/GSC have a kind of melancholy I really like that's similar to BW2, too. ORAS had surpringsly good writing after the shitfest of XY, which was a mess of ans idealised France, characters that constantly suck your cock and an attempt at a deeper story that went nowhere, but RSE beats it out in this regard because all the handholding makes it feel much less like you're on an adventure through a wild region. Kanto with gen 3 graphics really showed how bare it was, I felt, but Sevii islands were fucking amazing.