So since the 3DS eras-
XY: Cute. Adorable and actually really fun. The post game is non-existent(holed up in Kiloude city outside of Looker) though... Think we were supposed to get Pokemon Z but it just never happened. Also still the Pokemon game with my favorite outfit choices and just so much nostalgia cause 3D Pokemon.
ORAS: I never played the originals before hand and when I tried them years later on my pc I really, really disliked them. The only thing they really had over ORAS was the post game(again, Zinnia was crazy). They really evened out all the worst parts of the originals though and they also improved the mechanics they brought from XY(hoards in rain!!).
SM: First time playing what you'd call a base game I guess. Started with Plat when I was 7, never played BW but played W2 the year before XY. SM was just super barebones and you could feel it. No post game areas, not even a post game story.
USM: Better story, less dragging your feet at the start, MANTINE SURFING (!!!), festival plaza improvements, ultra wormholes, rainbow rocket.
Sh/Sw: The DLC was more interesting than the main story :/ You had access to the endgame from the start basically and farming raid dens was fun for a little bit. Then the randomness and reset heavyness of it all got boring. Then people started hosting specific raid dens and it was fun for a while again. Crown Tundra > Isle of Armour. Snow, ancient ruins, regi temples, catching all 4 swords of justice(cause they actually put Keldeo in the game), and Calyrex were all awesome.
Diamond/Pearl Remake: Art style didn't bother me but plat was my first game and this felt really bad for the most part. Why can't I get an eevee in hearthome city? what did you do to Gardenia and Fantina's gyms? Oh so this is where the no fire types meme came from... Oh right, no battle facilities or little battle hut where I can battle all the gym leaders again.
PLA: Fresh and new. Needs more battles and the battle system itself could use a few touches but it was fun. Kind of sad that the only real post game activity is shiny hunting. Also it's not really fun to replay. Your team just doesn't matter that much for the most part because so much of the game is focused around catching or dodging.