In terms of pokemon designs, definitely gen 1. I'm not one of those people who think everything after gen 1 is trash, especially since I think gen 5 is incredibly creative and the gen 6 and 7 rosters have very few misses, but I have to say I understand why some people say pokemon don't look like pokemon anymore. I've been reading some online discussions about it and it comes down to how pokemon are either more and more cartoonified (with round heads that are huge in proportion to the bodies or more humanized to look like characters rather than species. Gen 1 looked like actual monsters, with fangs, claws, pointy ears, ruffled fur, etc. This is not to say that later pokemon don't have that same vibe: see Aggron, Haxorus, Lycanroc, braviary, etc. but it's more of a general trend. Also many gen 1 pokemon have been tweaked to become less "monstrous" so it's more about the artwork at the time than the pokemon itself
In terms of the games' look and feel, I would say gen 2 for its retro simplicity and minimalism and great sprite work, then gen 5 for the overworld which is amazing to explore (I dont love the pixellated sprites but I have to say that they have a lot of personality, moreso than the 3D sprites)
For promo artwork, gen 1-2's watercolors are hard to beat!
