There are many niches within chip collecting, and each one has its "key" pieces. In the pre-casino era, Ivory chips and Crest & Seal chip hold special places. Same with key pieces from Europe, Caribbean ... even non-Nevada areas (Atlantic City, Colorado, Deadwood, etc.) have uber-desireable / nearly unobtainable pieces.

Having said all that, the center of much of the collecting world is Las Vegas -- either downtown or the LV Strip. And certain older properties (think 1940's and '50's, up to the early 60's) are in high demand and ultra-low availability.

Below are a handful of chips that are unique (Golden Goose $5) or VERY VERY hard to find and VERY VERY expensive. These are all $5 chips, with 1 exception (the $1 Showboat chip -- a beautiful designed chip!). There are equally "killer" chips in the $25 and $100 range too.

If you want to do additional research, get a copy of The Chip Rack, and turn to the back (Table D-1) for the population of "Z" valued chips (=$800+ for Z-1's; $21K-$99K for Z-8's). There are 6 Z=8's listed and over 1000 "Z" chips overall in Nevada, according to TCR. But, you will never likely see the most rare chips in person during your collecting lifetime, IMO. The hunt is fun though!

An additional resource on rare chips is the Campiglia book -- "Official US Casino Chip Price Guide: 4th Edition". Dated now, but you can also see references to rare / valuable chips by going through it.

Just FYI: The pictures below all come from MOGH / ChipGuide website, and are owned there. Not for re-publication elsewhere. It's a great website for exploring chips too!