Didn't include games I haven't played (started in gen 3, so skipped gen 1 and 2).
Gen 7 as a whole was hugely disappointing for me, so much so that I couldn't motivate myself to complete the games, and I didn't even bother with the sequel. I know Pokemon games as a whole are becoming easier, but the hand-holding in this game is atrocious, and so annoying. My idea of a Pokemon game is exploration with my team, and being resourceful with my team, so being constantly forced to watch cut scenes, and constantly running into road blocks was too much for me.
D tier, wouldn't ever revisit this game.
Gen 5 had some cool new toys, such as the shiny dragons post game, and the interesting plot development with N and Team Plasma. When comparing it to the other generations though, it really is in a league of its own (being sub par). Pokemon Sword is a good game, but I hate the approach Gamefreak have taken with cutting Pokemon and releasing DLCs for cash grabs. I think its a disgusting practice, and I only really associate this behavior with S&S. No updated animations is also laughable, they're reusing the same models we've had for what feels like 6+ years.
C tier, sort it out GF / barebones.
LGE/LGP were sold as a very barebones game, which I was fine with personally. It was refreshing revisiting Kanto with updated graphics, and having certain Pokemon follow you around like Charizard was a really nice touch. Eevee is adorable, and I didn't mind the new Pokemon catch mechanics. Pokemon candies were stupidly broken though, and lack of post game made this a "one and done" adventure. FR/LG are in a similar fashion, but when comparing to LGE, I personally didn't like the Sevii Islands, but the extra content is always welcomed. D/P were good games, and I like the majority of Pokemon in the regional dex. The majority of new evolutions for Pokemon such as Sneaseal and Rhydon imo were a huge success, and they really fit nicely into their respective families. The biggest downfall for these games is pacing. These games are so slow! The text and battle animations drag on, and that's not even including the tedious task of climbing through snow to reach Snowpoint City.
B tier, good games, would revisit.
I'm probably looking at R/S through nostalgia glasses, since they were the first games I played, but man I dumped so many hours into these games as a kid. 2 Bicycles that can perform tricks, Pokemon contests, hidden bases, and a ton of exploration make these have a special place in my heart. I view OR/AS in a similar light, as imo they stayed true to the gen 3 games, with some nice extra inclusions, like flying on Lati twins and better graphics (ofc). The lack of the game corner sucks, but nothing they could really do about that. HG/SS were superb games, packed with a bunch of content and neat little features that other games lack (constant follower Pokemon, ability to listen to old themes). Battling Red at the top of the mountain also feels like a true last boss, something other games have tried to replicate (Steven Stone in Emerald), but there's just something special about this moment. Pokemon Platinum has all of the good points of D/P, but offers loads of endgame content in the form of a battle facility, and feels like the "complete version" of the games.
A tier, great games, fond memories playing them.
Pokemon X is just fantastic. The transition from BW2 > XY in terms of style, graphics, and direction was huge, and almost all new features and changes added were positive. New Pokemon models, Roller Skates, introduction to Fairy-types, ability to ride Pokemon like Rhydon and Gogoat, the list goes on. Getting 2 starter Pokemon was also super interesting, and who doesn't want an extra starter? Mega Evolutions are best described to me as epic. The lore surrounding them, along with the change in design and animations made it feel very special. This is exactly what Pokemon games should be. Forming connections, innovating, and huge improvements over the last generation. I personally prefer rivals that are more bad than good, as it feels good beating them, but I didn't mind having 5 friends along the adventure. Only bad thing I have to say about X/Y is the post game content. Emerald is the complete version of R/S, and offers a bunch of extra content too. Instead of facing one bad team, you face both Team Aqua and Magma, which makes a lot more sense lore wise. Also, the ability to capture all 3 box legendary Pokemon in this game, along with the countless hours you can spend trying to get the Gold Medals at the Battle Frontier make this the best Pokemon game imo.
S tier, the best games GF have made due to their innovation and content.
