Herod..... Apologies for being a pedant. You cannot qualify the word "unique" i.e. "most unique" is incorrect. Unique means, one off, on its own, alone. If there is more than one it is NOT unique, less than one means it is only part of one: incomplete.
When it comes to aircraft types there are some obvious examples of truly (hey, fell into the trap myself !!!) unique examples. Spruce Goose would certainly qualify, as would the Reid & Sigrist Desford. Then there will be some that are unique, in that they have a "one off" engine installation but there will be other airframes with different engines.
Sometimes a particular Mark of a design will be a "one off, unique". The de Havilland 106 Comet 3 was unique but of course there were others Comets of different Mks.... While we are on de Havilland aircraft, the deH 81 Swallow Moth & deH 92 Dolphin were both unique.
Last edited by Planemike; 21st Mar 2022 at 12:58.