Then if you know multiple languages, the most useful will be the one you can use the more often, on the various platforms you have to write programs for, and for the various kinds of programs you have to write.
In this respect, I find Common Lisp particularly useful, since it's really a multi-paradygm programming language, in addition to being a meta-programming language: when you need to solve a problem, a feature that is not provided by the language, in Common Lisp you can usually very easily add this features thanks to macros.