Best wire to connect amplifier binding posts?

Use the following link to figure out gauge:
http://www.csgnetwork.com/wiresizecalc.html

This link has minimal voltage 120V, but all you need to know is output power of your amp.

For example if your amplifier delivers 200wpc, you should specify 2 amps. Use the following Ohm's Law formula Power = product of Voltage and Current thus specifying 2 amps you figure out gauge for 240Watts.

If you have large power amp to avoid large gauge of single conductor i'd recommend to use stranded wire to avoid skin effect. The skin effect tolerances you can view from the following chart: http://www.powerstream.com/Wire_Size.htm and if the tolerance level is high for a specific gauge you'll calculate, using single conductor wire will be more efficient vs. stranded but just by very small notch.

RW is partially right with exception to wide bandwidth amps that may actually suffer from such.

Dtc you are absolutely correct. Fancy wires are expensive just because they're exotic, good looking and certainly not because of engineering or cost of materials. Adding a dedicated oak box with some exotic company logo will make them even more fancy.