Quite a bit is getting difficult to get hold of now, but for manufactured items and not prototype / rider only issue parts it has to be Profile. They made some of the most hard to get hold of items in my opinion.
Not often you see the - Chrome Profile / Scott Pad Set, Horseshoe Pedals, Hutch Gen 1 Cranks (not Aerospeeds), Handle Bars including Freestyle, Lock Jaw Stem, Gen 1 Box Cranks and Coloured Spiders.
J.