'Best' means different things to different people. Highest level of protection? Most comfortable? Most durable? Etc. Some might say that the 'best' pair of chaps is a pair of chainsaw protective pants!
As I understand it, the US standard sets a minimum level of protection, so chaps either pass or fail. No distinction for being 'more protective' or protecting against a more powerful chainsaw. In Europe, the pants (they don't like chaps) are rated 'A', 'B', 'C', and '1', '2', '3', with 'C' providing the most coverage of protective material, and '3' protecting against the higher chain speeds.
But I also want chaps that fit, are comfortable to wear, hold up to use, are easy to maintain, and are affordable!
Phibert