The Flaffy/Mareep/Ampharos is really rare for me, plus Topegi/Topogetic; that’s of the evolvable families. Snubbull/Granbull was also pretty rare.
Of the non-evolvable ones, Unown, Heracross, Corsola, Delibird, and Smeargle I have yet to catch. Seen one Heracross on a Gym, never even seen the others. Seen one each (on Gyms) of Tauros, Mr Mime, Kangashan, but not Far-fetch’d.
I notice that soemtimes really rare types become common for a week or so in one area; and also some areas always seem to be populated by certain types. For example I’m in Hawke’s Bay, and Napier always seems to have a lot of Magnemite and Voltorb’s around the CBD, but they’re very rare elsewhere.