For Me.......
....because my two special areas of interest are poker room chips and prop chips, a set of the UPC Flying Comets used in "The Cincinatti Kid" would be "IT" for me. These were the chips used in the last scene where THE MAN (E.G. Robinson) makes his miracle hand. I had some good contacts/leads (I knew a guy who knew a guy) and made a few calls but made no headway. They treated props differently back then, and because it is a common chip, we would have to be very lucky for them to turn up and be able to prove they are the ones. Probably in the garage of the nephew of the prop guys second cousin, being used is a .25/.50 cent game!
Honestly, though, I guarantee you, if I ever got them, the day after there would be a new "Holy Grail" chip to fiend over. A Bugsy Flamingo, probably. That is why I love this hobby!