It's hard really hard to say exactly what is the 'best' programming language to learn. Programming languages are just like tools and they were created for specific purposes in mind. It's like pulling out your toolbox and asking, "what's the best tool in this toolbox?" Best for what?
As far as usage goes in the SEO / web world, I would suggest Ruby. Ruby is an interpreted language that has immense community support from the dev community. It's adopted to work with Rails (Ruby on Rails) which is arguably the most popular web framework right now.
In addition, Ruby works very well for writing one off scripts. It's a spiritual successor to Perl, a previously popular programming language for writing scripts. If you know Ruby, you'll be able to read through a lot of legacy Perl scripts as well.
However, as I mentioned, programming languages have their specific time in place. For instance, if you wanted to build a desktop application, you probably wouldn't be looking at Ruby and probably looking at C# for windows or Objectitive-C for Mac. Or, if you wanted to build something computation heavy, you'll be looking at something more efficient like C++.