What are the rarest Pokemon Go moves?

It is Poliwhirl's unique move in PoGO, but in main games tons of water type pokemon can learn it.

I have a togetic with Steel Wing. Not a great move (and not rare), but it became legacy before Gen 2 was fully released.

Transform is only available to ditto, but it is not legacy. Another rare move is brine on omanyte, which is legacy.

Rumours say that there exists trainers with an Omastar RT+RS. But it's close to a myth.

Cut is only available on Sunkern currently and is legacy on Farfetch'd.

Sand Tomb
Magnet Bomb
Horn Attack
Flame Burst
Flame Wheel
Power Whip
Shadow Punch
Dragon Claw
Aqua Jet
Sludge
Draining Kiss

Thanks, also found one with scald, hoarding is good sometimes

You can always TM moves. The rarest moves is the one you cannot get anymore.

I have a Chansey with Psybeam.

Considering how rare Chansey is/was for some, I can't imagine there are many around.

As a bonus it is lvl1

The rarest good moveset is a Omastar with Rock fast attack (became legacy quickly back on the day).