It's difficult to pick only one, but I'd have to say Ruby. The syntax is wonderful to work with, I can often implement the same logic in fewer lines of Ruby than other scripting languages I find, and it has constructs that make for elegant code like metaprogramming. It's certainly not the fastest language out there and those same concepts like metaprogramming can be abused, but ultimately I find it to be an excellently designed language that's a lot of fun to work with.