Most expensive today, or most expensive in its day?
Rarest now, or fewest produced commercially?
If you are talking about antique motors, perhaps the Gray Gearless commands the highest prices. Modern day, no contest, Seven Marine 627.
Most expensive in its day, the Riley (?)
Dunno the value and rarity of the Motogodille but it ought to be up there. Submerged Electric ditto.