My list of "real rarities" includes, but is certainly not limited to:
Campagnolo items: Hi-Lo rear (and front) hubs - Super Record titanium hollow BB axles and hub axles - Belt buckle boxed sets (less than 1000 of them compared to 15,000 anniversary sets) - Campagnolo coaster brake rear hub - NR/SR "Century" parts - Large square "Continental Divide" Campagnolo belt buckle by Bergamot Brass Works (likely even fewer than 1,000 out there)
Regina: Record Extra aluminum and/or titanium cog freewheels (NOT Titall, these predate the Titanio model, some have steel Extra bodies, a few have aluminum bodies) - Titanio chain
Others: Pino Morroni components/bikes - Hi-E frames (only 15 ever made) with the matching cranks & 200g rims - Hi-E seat posts/handlebar stems (never seen these two other than prototypes) - Mossberg VM-I/X-1000 "graphite" frames - Speedwell Titalite frames - Shimano Dura-Ace 10 track hubs with unique small diameter cog threads
Honorable mentions go to (among others) Shimano Dura-Ace centerpull brakes with "safety levers," long-cage Crane derailleur (NOT GS) and the remaining parts in the Dura-Ace 10 track group. Arnold Engineering and OMAS Bolt kits and PMP cranks also come to mind, as does the "starfish" Mavic crank, nearly impossible to find un-scratched...
The above list is merely my own based on experience, YMMV. Other items may also qualify... ideas?