Gen 1 started it all and boasts what are my favorite remakes in FRLG. It also has my fav remixes (I especially love the Lake of Rage theme, it's amazingly done).
I prefer vanilla GSC to HGSS, mostly due to music remixes being a mixed bag (to me) as well as the "chosen one!" narrative in the remakes.
Gen 3 brought us the prettiest region in Hoenn. The area between Mauville and Lilycove is my favorite in the series. Emerald also let you fight both bad teams and introduced the best Battle Frontier.
DP gets a bad rap due to running exceptionally slow and having such a limited pool of available Mons. Most Gym Leaders and Elite Four members don't even have teams that share a typing (Flint is a good example). Those issues were fixed with Platinum, which introduced such a good selection of Pokemon and gave us the Distortion World as well.
BW is probably the most bold game made by Gamefreak due to the all-new Pokemon and impressive story. BW2 had a less impressive story and offered a lot of cool changes, such as new availability of really cool Pokemon and revamped gyms (I think confronting Elesa on the runway is my favorite moment in the entire series).
XY is fairly ho-hum compared to other entries. It was oversaturated with too many Pokemon and too many locations, to the extent where a city with a giant crystal becomes unmemorable imo. At least it gave us Mega Evolutions (which I think should've stayed there imo), easier breeding mechanics and the Boutique music.
Gen 7 gave a different take by switching out Gym Leaders for Trial Captains (with their respective trials), Totem Pokemon and Kahunas. It also gave us a fantastic cast of characters, including Kukui, the lovable Team Skull, Guzma and Lusamine (the undisputed best villain in all Pokemon).