The rarest Pokémon of them all in the series.
Nah Mew was not that rare. Everyone knew how to get it and it wasn’t about chance really, just strategy and how to get all the Pokemon back then.
I am sure there are many other Pokemon in newer generations that are much rarer.
For Pokemon GO it is currently uncatchable.
That’s right. Spinda.
Spinda’s unique spot patterns change with (nearly) every encounter. Because its design is based off of its personality value (a 32-bit integer created as the Pokemon is encountered)*, the chance of finding any specific Spinda is 1/4,294,967,296.
Making it, by far, the rarest Pokemon one could hope to catch.