Some are extremely rare because there are very few known examples. There is a Joe Wells chip where his name is in a red hot stamp rather than printed in black. To my knowledge, there is only one Joe Wells red hot stamp chip. Other examples include:

Karl Maier - showed up on eBay within the last year. Went for almost $900.

Pat Foust - sold on eBay for almost $1,300.

Larry Rosen - two examples have sold on eBay for $500 & $600.

Martha Herndon - to date, only seven are known to exist.

Ed Lewis - to date only two are known to exist.

Now, compare those to Morrey Brodsky and Bob Coyner, where 2,000 were made of each chip.

I've never seen a Jack Walsh chip or a Bill Deer chip. But that's part of the joy of collecting. Because you just never know what will turn up.