there is no such thing as a general "best" string material for any given bow. ditto's for a "best bow" or "best arrow" or "best broadhead". see where this is going? :D
there are essentially two types of "modern" bowstring fibers, polyester ("dacron" - b50, b500, b55, etc) and low stretch/low creep hmpe ("high modulus polyethylene" - dyneema, spectra, vectran - fast flight, d97, 450, etc).
almost all modern laminated stickbows are designed to safely handle hmpe bowstring fibers, which includes howard hill archery longbows.
the bottom line is that *you* need to try out different string fiber types and choose what appears to work best for you.
my opinion - i only use hmpe bowstring fibers as they allow using less strands (polyester/dacron will not allow this), for a better feeling release/shot. there are some who find less strand counts contributes to a quieter bow on release, and faster arrow speeds.
again - howard hill bows can safely use any modern bow string fiber, and it's up to you to decide what will work best for ... you.