Do you mean the same snake or what is the rarest and then what is the most expensive?
To be honest there are a lot of "rare" morphs both base and combo. Every year there are more and more "new" combos produced, which would make them (for a little while) one and only morphs. Plus there are plenty of morphs that have yet to reproduce making them one and only morphs.
As far as most expensive, that's tough to say because what you see advertised on KS are *NOT* the only snakes up for sale. There is plenty of behind the scenes action going on, so who knows what snake is being sold at what price. Also keep in mind asking price is not necessarily the selling price. Plus every year the prices change, Lessers use to be selling for $30,000, now you can pick one up for $400.