Interesting question... I would have to say that there are a few categories to this answer:

** All images courtesy of Chip Guide **

1. What chip do I know exists but I don't think I have a shot (or the money) to get? I'm going to go with the $5 Golden Goose (CG030413). Hearing that this chip sold for $75k at a previous convention was crazy. With a book value of Z8, it was in the range, but still.

2. Chips I've never seen offered? $5 New Boulder Club (CG043500)

3. Chips I've seen offered from time to time that really catch my eye, but price was just too high? I'm going to go with the green $5 Big Bonanza (CG040892). Absolutely lovely chip, and I've been offered a gorgeous specimen at $3500, but I just can't pull the trigger at that price.

Honorable Mentions:
$5 The Northern (CG199999)
$5 Sinabar (CG065734)
$1 The Willows (CG114923)
$5 China Mint (CG140626)
$5 Alpine Club (CG162537)