[thesuperbigfrog]

Depending on what you are prototyping I would recommend Python, Ruby, or Perl.

Python's "batteries included" approach and numerous libraries make it easy to build prototypes quickly.

Ruby (and Rails) have been used to create the first generation MVPs for many startups in the past. If you have Ruby experience, it's a good choice for prototyping.

Perl is good for prototyping command-line and system tools. The early versions of Git were partially-implemented in Perl.