Polyurethane Surfboard Foam has been the industry standard ever since Hobie Alter of Hobie Surfboards and Gordon Clark of Clark Foam began experimenting with it as a replacement to the heavier and harder to work with Balsa Wood Surfboards. Since the closure of Clark Foam in 2005, other materials are starting to make head way, most notably, EPS Surfboard Foam.
However, PU Surfboard Foam still holds as the industry standard as it has unmatched flex characteristics and is so versatile in it's application and the resins that can be used with it. Compatible with either Polyester Resin or Epoxy resin, PU Foam is still the most versatile. But, look for shapers blending these materials like DHD's Epoxicore construction which combines both PU and EPS foam for a board with unique flex patterns where you want them.