One of my favorite molds is the rectangles and hearts mold shown here on the Wilbur Clark's Desert Inn $5 chip from my collection. One of the most popular molds is the arrowdie. No matter what area of interest a chip collector might have, they always seem to want at least one chip with the arrowdie mold in their collection.
Quite a few collectors try to obtain an example of every mold known. That will keep you busy for a while!
The best place to see and learn about molds is at Robert Eisenstadt's great Web site. The link to the mold design section of his site is below. Make sure to bookmark that page, you will refer to it numerous times over the years, as you acquire more and more chips for your collection.
http://www.antiquegamblingchips.com/molddesignindex_site.htm