At the moment, it is my opinion that the top 4 rarest Pokemon (in order of rarity) are:
I’ve never seen a Lapras in the wild and have only caught them in 10km eggs. The theory that Lapras is the rarest Pokemon is also supported by Jeremy Lai (the guy that was the first to catch all the Pokemon), according to a new report I read.
Other rare Pokemon include Aerodactyl and a few others.
Since Gen 2 came out, I now think that the rarest Pokemon is Unown (one member on this forum mentioned that the had been a couple caught in Christchurch), followed by Porygon, then I guess the aforementioned.
Girafarig also seems rare to me, I’ve only seen (not caught) one.
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.
And just to add, have seen and caught 1 Lapras in the wild (at least I think so, was a while back), only ever hatched Snorlax. Seen and caught 2-3 Chansey, hatched a couple more. Didn’t have any Girafarig until about 5-6 weeks ago when they were everywhere on Te Mata Peak one day, must have seen 10 or more.