I'm going to have to go with the Conan ID (not for completely selfish reasons I promise
) I've seen many more store dekas and devilgotchis for sale than the Conan... 100 were made I believe (though english websites and Wikipedia will tell you 500). From what I've been told, Conans have been sold for up to one grand.
While I think the Conan is the rarest tamagotchi version, there are many shell designs of well-known tamagotchis that are rare. Check out the special Tamagotchi Plus editions on this page:
https://www.bladesplace.id.au/bladeagotchi/tamplus-limited.html. Here are some rare vintages:
https://tinkerville.cutthatout.com/tamagotchi/rare/. ...and these are just a few examples!