"RAREST SIGN"---pretty broad generalization.
Do you mean the most expensive?
The best condition?
The one that there's only "ONE" of?
Or the one that no one has ever seen, but only "HEARD" about?
Generally, collectors equate value and rarity on the same plane, and they don't feel a one-of-a-kind sign is very rare, unless it has a lot of value?
Lots of variables here that could be debated for years with no conclusion.
I have a Cities Service sign that seems to never have been seen before and its priceless to me because a friend secured it for me! Is it the rarest of the rare? I don't think so, but a lot of collectors will never get to see it unless they venture to North Dakota!
I'm sure there are rarer ones though!