Related Clues:

• Most seldom seen
• Least likely
• Hardest to come by
• Like a numismatist's greatest treasure
• Most like a collectible
• Most in the raw
• Most difficult to find
• Least plentiful
• Most uncommon
• Superlatively collectible
• Least known
• Most unique
• Least spotted
• Least well done
• Most coveted by collectors, perhaps