GSC / HGSS are my favourites. Closely followed by BW (1 not 2).

Pokemon Crystal is my favorite one because it reminds me of being a child

I dropped out of the series at Ruby. A friend convinced me to pick up the Diamond remaster and give it a crack, and it was...alright. I think I left off right before the Elite Four, but I'd been kind of chugging along for a while more out of a desire to do battles with other people than anything else.

The last one I actually enjoyed was probably Silver. I think it might be the only one I've ever beaten, for that matter.

black and white,
only pokemon game I ever 100% the dex on was pokemon white, fucking great game

Black and White was actually really cool, it was interesting to see the pixel art get some life to it. Felt like the sprite artists were at the top of their game.

In retrospect I wish the series would've just stuck with those static-ish images and saved the complex 3D animations for attacks. The main difference that sticks out to me in older Pokemon games is that my mind fills in the blanks and can imagine the characters doing cool things during battles, whereas the fully-animated models leave nothing to the imagination and the animations themselves are kind of soulless