In bullet points! Also I'm including the remakes in the group of their original names, although I'm not taking the remakes into account that much.
6. Gen VI (X and Y)
-graphics were pretty amazing.
-fairy type, while not executed perfectly, overall did it's job of nerfing dragons and steels a bit.
-I liked the new moves it added.
-some new features that I liked.
-I actually liked most of the new pokemon, there just weren't enough of them, which leads me to...
-The game spending all of it's time on Megas and not working on it's new pokemon at all, resulting in a tiny portion of new pokemon, replaced with OMG MEGAS!
-speaking of megas, mega distribution was terrible. Instead of giving them to pokemon who...actually needed help, what did they do. two to fucking mewtwo, two to charizard, and most of the megas all went to the already overpowered pokemon like Gengar and Lucario which didn't even need help in the first place, just to appease the fanbase.
-The developers desperately trying to appeal to older fans by making it as much like Kanto as possible, rather than actually make a good game. let's see: all of the kanto starters got megas, most of the megas went to kanto pokemon, the rest went to mostly fan favorites, the region is shaped like Kanto...yeah, not a coincidence here.
-Most of the megas are ugly as sin.
-For a new type, the new fairies that came out were very weak and unusable competitively.
-The mewtwo bias is real.
-EXP share made the game way too easy unless you turned it off.
-the story took a huge step back compared to black and white.
-the rivals mostly doing nothing at all.
-In general, while I appreciated some parts of it and I still love playing pokemon competitively, I did not enjoy X/Y and it's attempts to pander to the base rather than doing anything interesting or risky.
5. Gen II (Gold Sliver Crystal, Heartgold and Soul Silver)
-Added two types that were very needed (dark and steel) and nerfed psychic.
-Some creative new pokemon.
-made several of the still-most influential pokemon in the metagame today (yeah I do competive).
-Heart Gold and Soul Silver were actually a huge improvement.
-Red boss battle<33333
-Silver is a great rival and I wish we had more asshole rivals now a days.
-Ghetto Kanto with everything removed or torn down was seriously a chore.
-Game was way too easy, level curve was terrible.
-Two regions is in general too much.
-A lot of bad/forgettable pokemon.
-It's a bit too similar to gen I, not really much new.
-it's just kind of...dull, especially just reusing team rocket lol.
-it offered some new improvements to the franchise, but on it's own it's probably the weakest pokemon game.
4. Gen III (Ruby/Sapphire/Emerald, Omega Ruby/Alpha sapphire)
-Good difficulty rating; the game actually does get hard at some points.
-Gym leaders all work really well and are fun to fight/defeat.
-the land area of the region is fantastic.
-The first pokemon games that actually got epic.
-Emerald's level of difficulty increase and new content was something unique.
-Adding some ghosts/darks/steels after Gen II introduced them.
-Emerald Battle frontier will never be replicated in awesomeness.
-lots of post-game goodies.
-TOO MUCH FUCKING WATER. Half of the region is surf routes with swimmer after swimmer after swimmer.
-Due to the water being everywhere, there's not a lot of variety in what you can catch.
-aqua/magma's plans really make no sense whatsoever lol
-that's...basically it, really. I really like Hoenn, but those flaws are crippling imo.
3. Gen I (Red/Blue/Yellow, Fire Red/Leaf Green).
Honestly I can't really judge this one because it's such a classic. It's just something totally apart, and you can't rank it without nolstagia. It's still great though.
2. Gen IV (Diamond/Pearl/Platnium).
-Probably the most existential and epic pokemon's ever gotten.
-I actually like all the legendaries in this game lol, they're all great and unique.
-Sinnoh being rainy and snowy was a huge departure from normal pokemon regions and I really liked it.
-Mount Cornet was one of the few places in a pokemon game that was legitimately long and exciting. I actually wish pokemon would do more mountains like that.
-gave new evolutions to pokemon who really needed them, and made a lot of them very usuable.
-physical/special split changed the game for the better.
-I really don't like Barry.
-May have overdone the legends a tad, but I really like most of them so idrc.
-this however, was the time where pokemon started really overdoing the events...eh.
-great game, I love it, super underrated.
. Gen V (Black/White, Black 2/White 2):
-Best main-game storyline pokemon has EVER had.
-Gym leaders are all fun and all contribute to the plot.
-legendaries and their roles are just perfect this time around.
-having no old pokemon was honestly a great move and I loved it, especially since with a few exceptions I thought the new pokemon were really creative.
-elite 4 in any order, I like.
-decently challenging game.
-I don't know I just love this gen so much ;_;.
-not a lot of post-game, and that's pretty minor. Can't think of anything else.