>S tier
HGSS: Hours of gameplay, big postgame content, two regions, countless minigames with the triathlon, reintroduces walking around with your Pokemon. Best game of the series, standout being two regions and overall colorfulness.
>A tier
DP/RSE/B2W2: Excellence in storytelling, introduction of great mechanics like contests and bright characters. Gen 3-4 have the best music in the series and expanded focus on exploring the world and lore surrounding Pokemon.
>B tier
Pt/FRLG/RB/GSC/ORAS: Solid games with high replay-ability factor
>C tier
XY: Solid and landmark for their introduction of a new typing and Mega Evolutions, however they lack replayability factor in that there's little postgame content and encourage little exploration of the overworld. The characters are lacking charm and personality.
>D tier
BW: Too linear to be enjoyed. Introduction of seasons was a detractor that it made it difficult to access after putting it down for a period of time. Introduces the most original storyline in the series and some of the most memorable characters, but lacks charm. While the Pokemon in this game were some of the best designed in the series, many others suffered from being ugly and unoriginal. More fun to think about than to actually play.
>F tier
SM/USUM: Terribly long cutscenes, unoriginal storylines, painful linearity and turned Pokemon into a very childish game with introduction of trials. Great postgame content, but that couldn't save it from its mess of handholding and overproduction. If you didn't care about the story, it becomes painful to replay due to its amount of cut scenes and corny dialogue. Takes the series to new places, but reminds fans some places need to be stayed away from. And for an island lacked a certain amount of color and charm. Characters were mediocre, especially the characters of the main story line, who were not tropical or pacific in the slightest. Almost grotesquely pale in this sense of storytelling, marred by lack of depth and cliches.