Lapras is extremely rare here too, despite that everyone around me seems to have them in droves. I was lucky enough to catch one at the beach a few weeks ago, though.
My hardest-to-find is probably Aerodactyl, Lickitung, or Grimer. The local biome just doesn't support them spawning here. But Aerodactyl is the rarest of them all for me. I've at least seen Grimer and Lickitung on my radar once, even though I've never caught them. That isn't the case for Aero, unfortunately.