it might be a weird pick since its one of the more recent ones but i have a ton of nostalgia for pokemon moon... i was sort of Going Through It around the time it came out and would play it to chill b4 i went to bed.. its just rly relaxing and nice

also idk if they added it in that gen but i love the mode where u can pet ur pokemon and feed them little jellybeans lol

I'm into gen 2 and especially crystal. The GBC limitations are used to their full extent. The well-chosen palettes combined with excellent dithering and texture really appeals to me. Plus I like the rural vibe of Johto, especially compared to how high-tech other Pokemon games tend to be. You learn how pokeballs get made, you visit the local radio station, there are owls and bugs and stuff, it feels like a camping trip in a way no other pokemon game does. It feels like it's set in the seventies with cellphones. I love it. I also love the oft-discussed cutesy designs... Sneasel, Celebi, Umbreon, theyre all wonderful.

Contrastingly I love gen 5, for just about opposite reasons. I love seeing a representation of the place where I grew up in a video game - especially because it's not just a few moments spent in Manhattan, the whole game world is set in that familiar NYC & North Jersey area. And everything in gen 5 is so utterly stylish - the speech bubble text boxes, the angular and cool GUI. Plus - and this is totally bias - there are so many pink-and-black Pokemon, and i LOVE that color palette. Scolipede, Zorua, Hydreigon.

Gen 4 babyyyy It was the first gen I actively got to play ^^ Chimchar is still one of my favs of all time and the legendaries just rattle my brainspace (Love u 4ever Dialga)

gen 4 hands down. 
for me personally nothing can compete with the sinnoh games (also the hgss we're technically part of the gen 4 games too and those were also incredible) 

alola!! i know not a ton of people love alola but it was my first pokemon game on the 3ds and i have so much nostalgia for it :-)