For my money, Ruby comes closest to realizing the "promise" of dynamic languages. That is, it's fast to build with when prototyping, flexible when refactoring,

For my money, Ruby comes closest to realizing the "promise" of dynamic languages. That is, it's fast to build with when prototyping, flexible when refactoring, reasonably fast enough and not so memory hungry that it can't handle "web scale". Obviously there are languages that are better in each of these areas, but few that combine so many as well as Ruby. In other words, if you know that you'll be working in a problem domain (e.g. high concurrency) where specific advantages of some other language (e.g. Go/Erlang) are likely to be needed, use that language. If you're just building something, then Ruby's a fine place to start.