Gen 5 > Gen 4 > Gen 3 > Gen 6 > Gen 7 > Gen 8 > Gen 2 > Gen 1
Honestly tempted to put Gen 8 below Gen 2, considering Gen 2 added a lot of content from Gen 1, genuinely improved upon graphics, fixed many of the issues from Gen 1, etc whereas Gen 8 hasn't really done anything to make it any better than XY and USUM/SM but Gen 2 has a lot of issues with pokemon distribution, level scaling, etc that holds it back.
Gen 1 is objectively the worst game, but this is mostly due to being the first game resulting in awful sprites, bugs, lack of content, etc. I can't, in good mind, put it on the level of any other generation but it is understandably a result of being the first game that every other generation has improved upon.
I honestly didn't care much for Gen 7, and haven't played Gen 8, so I am rating Gen 8 below Gen 7 due to the lack of postgame content, dex cut, etc. I also don't like Leon or Hop from what I have watched thus far but my opinion may change on them.
Gen 6 has excuses as the first mainline 3D game, first to have customisation, etc but the reason I am rating above Gen 7 is due to generally less interruptions and lack of experience with the Gen 7 games. It seems like Gen 7 objectively has better and more postgame content an interesting story and more pokemon, but I'm not a fan of most of the pokemon in Alola.
Gen 3 is good but there is nothing that would make it anywhere on the same level as Gen 4, barring Gen 4 not having Secret Bases.
Gen 4 is objectively superior to Gen 3 in everyway due to all of the upgrades, updates, additions, etc that it added to the franchise by using Gen 3 as its base.
Gen 5 pretty much perfected the formula with all of the side quests, postgame, animated sprites, seasons, regional design, writing, and so on. It basically took Gen 4 and made just about every aspect better. The only criticism I have ever seen for Gen 5, at that, is a dislike for the Gen 5 pokemon designs.
I strongly feel my opinions on Gen 1 to 6 are accurate, but I haven't really played Gen 7 and won't be bothering with Gen 8 so my opinions on them are mostly developed from videos, reviews and comparing them to the first six generations.