FWIW, I’ve caught all Gen 1 excluding the regionals and legendaries. I’ve seen everything except Farfetched. Gen 2 I’ve got most, all that you can evolve bar Ampharos, but not Unown, Heracross (seen 1), Corsola, Delibird, Smeargle, plus what I think are legendaries from Gen 2, Raikou, Entei, Suicune, Lugia, Ho-oh, and Celebi.
The Mareep/Flaffy/Ampharos family is the rarest for me, and Topegi/Topogetic. Girafarig was another one hard to find, then one weekend I caught about 5-6 on one walk.