How do you define "works the best"? Does that mean: "does not break", or "shoots the most pleasantly" or "shoots the fastest" or "most bang for bucks"?
If there was just one combination that is the best, everyone would be using that combination and nothing else. It's obviously not as simple as that. There are soooo many factors to take into account. I could advice you to use Mongolian tulipwood, but if you don't live in Mongolia this advice would be totally useless.
In general, the best performing bows require a design that matches the belly wood's mechanical properties to the backing wood's properties. Hence, a maple backed walnut flatbow has a better performance than a hickory backed ash ELB. Maple is also a better backing wood for walnut in general than hickory, since the higher tensile strength of the hickory can overpower the walnut belly. Some typical, well matched combos are bamboo/hickory backed ipé, maple/ash backed walnut, maple/ash backed jatoba.
