I wouldn't necessarily say "the best" documention, but both Pod::Usage and Getopt::Long have documentation that are spiritually akin to Perl itself - easy to get started with, yet no dearth of depth.   So much so, that even a lunkhead like me easily figured out how to use them* with a degree of effectiveness.
  cheers,
  ybiC
  striving toward Perl Adept
  (it's pronounced "why-bick")

* shameless plug