I would follow Tung's suggestion to divide the rarest models by brands. The first model that pop out of my head was this
Dragon Wings Sample East Touch B747-400 with 12 units ever made, still sold by Johanchan (I got best offer). I like the tilting gear on this model. Besides, DW B747 are also excellent by its overall shape too!
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For Aeroclassics maybe this
Aeroclassics Hangar Club HC002 Drax Air Freight Lockheed L-188 PP-DRX from 1979 James Bond
Moonraker. I got this without its original box nor its certificate. I like to have a variety of classic props for my diorama.
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