Since my previous attempt was not compatible with the rules of this sub, here is an updated version.
S-tier:
Firered/Leafgreen have all the qualities of Red/Blue, but with zero of the downsides. The Sevii islands are a great addition and my favourite region out of all the games.
Emerald is the definitive Hoenn game, with the full conflict between Team Aqua and Team Magma. Hoenn is my second favourite region.
Alpha Sapphire/Omega Ruby are equally well designed improvements of their predecessors, with better fleshed-out villains, and a great post-game addition, the Delta episode. Also Latias/Latios as the perfect alternative to flying.
A-tier:
Red/Blue/Yellow: The OGs. Outclassed by Firered/Leafgreen. Kanto is my fifth favourite region.
Gold/Silver/Crystal: Perfect sequel to the first generation with Kanto as the postgame. Johto is my fourth favourite region.
Ruby/Sapphire: Outclassed by Alpha Sapphire / Omega Ruby. Not much more to say.
Heartgold/Soulsilver: Have the same qualities as Gold/Silver/Crystal with lots of fun additions, but with graphics that I like slightly less than those of their predecessors.
Brilliant Diamond/Shining Pearl: Just Diamond/Pearl with better graphics. Sinnoh is my third favourite region.
B-tier:
Diamond/Pearl/Platinum: My first Pokemon games and great nostalghia.
C-tier:
Sun/Moon/Ultrasun/Ultramoon: Good region, but the games just aren't what they used to be anymore.
D-tier:
Black/White: The story with Team Plasma is the best out of all the games, unfortunately the rest about this game just sucks. The sprites look pixelated in battle, the region is boring as hell, lots of the Pokemon don't feel like Pokemon and it seems like in terms of graphics they were aming for things they could only do on a 3DS on a regular DS.
E-tier:
Black2/White2: Same as before, but this time not even with a good story.
F-tier:
X/Y: I couldn't find anything enjoyable here, and Team Flare just shows that they completely ran out of ideas.
by Lucyyyyyy_K