I did some Google searching, a couple times- it brought me back to this forum.
Funnily enough- The rarest Morph is the true WILD type.

Truly devoid of any designer morphs.
However, it is tough to say which is TRULY the rarest morph. Because breeders have projects in the workings everyday and have come up with new morphs which no one else has. Also, something that used to be rare (albino's) have since been mass produced, really nothing is too rare.
Formally- I believe it would be the Black Pearl.
There is the PieBald, which is kind of rare
The Galaxy - very expensive and looks to be only sold by Ron Tremper
Sunrise Blazing Blizzard- I have never heard of this morph! lol. Must be rare, as its only tied to one breeder.
all of the melanistics seem to be extremely rare,
I just found this cute lil guy who is a snow hyper melanistic
The dreamsicle is pretty rare!
I found a Mack snow tremper albino enigma for sale, for $100. I would say its safe to say they are not rare!
I found a Murphy patternless for sale for $65, safe to say- not rare!