Very fun question! I only collect fractional chips so this may or may not be the rarest chip in Kansas but it is the rarest chip that I am aware of. It is an orange 25¢ unicorn mold chip from Casino White Cloud in White Cloud, Kansas. According to the ChipGuide, this chip might not have ever been used in the casino and it's now a slots-only joint so it is probably unattainable.

(image courtesy of the ChipGuide)