Best first versions: XY
>Best regional Pokedex so far
>First 3D mainline games
>Trainer customization
>PSS
>Managed to have Triple and Rotation Battles
>Lots of shiny hunting mechanics (Pokeradar, consecutive fishing, Friend Safari)
>Best Pokemon interaction (Pokemon Amie)
>Competitive heaven (Friend Safari Ditto, Horde encounters/Super Training, easy grinding)
>Looker episode is still a nice postgame story
Best third versions: Platinum
>Fixed DP speed
>Improved Pokedex: now with more Fire
>Better Battle Frontier than Emerald's
>Cool postgame bonuses (lv. 100 Magikarp, Elite 4 rematches, your own mansion)
>Contests, Secret Bases, Wi-Fi Plaza, Game Corner, Underground and Poffin creation minigames
>Distortion World was cool
Best remakes: HGSS
>Most things that made Platinum cool return
>Pokeathlon
>Pokemon following you
>Many availiable Pokemon (Hoenn/Sinnoh sounds, various starters, updated Johto Pokedex)
>Safari Zone returning
>Fixed the "saving a lot of data" from DPPt
>Easier grinding for the postgame battles
>GB sounds were kino
Best sequels: BW2
>Unova updated
>Lots of Pokemon
>Challenge mode
>PWT and postgame city facility
>Story and characters have a cool developement
>Fast-paced battles
Honorable mentions for Ultra SM, FRLG and Emerald for being amazing updated versions, and GSC and BW for being really cool first versions. I chose XY because it usually gets compared to third versions and not original ones, and it gets very solid for being the first 3D main Pokemon games, although the story and characters could have improved quite a lot.
