I only played up to Gen 4 (I know shame on me) but here we go:
Good: The one that started it all, and probably the best set of pokémons for me.
Bad: Well really hard to hate something on Gen1 for me, but i have to go with the graphics, because honestly they didn't age well(Unless you count FR/LG)
Good: It was a great game on itself, but backtracking to Kanto and facing your old character was definitely the best part, literally a manual on how to make a sequel.
Bad: My favorite Gen so hard to hate here too, but if you count crystal i don't like how nearly impossible it is to get all three legendary doges.
Good: Beautiful design and GBA graphics, at the time i was just astonished by how amazing it looked. Also really well polished game.
Bad: Too much surfing.
Good: All the amazing new evolutions.