Just for shits and giggles, here is my list with no actual order by importance. Mine is more oriented on trying different paradigms.
1. C (the standard procedural language)
2. Smalltalk (the definitive object-oriented language)
3. Lisp (functional programming and historical significance, it pioneered such concepts as dynamic typing and recursion)
4. Assembly (low-level concepts, understanding of CPU registers and machine instructions)
5. Java (for enterprise purposes and to get a feel of the actual contemporary object-oriented paradigm)
6. Prolog (logic programming and artificial intelligence)
7. Python, Perl, Ruby or Tcl (any general purpose scripting language, preferably Python or Perl)
8. JavaScript (client-side web scripting, aspect-oriented programming)
9. Shell scripting + sed, AWK and bc (system administration, pipeline programming, general purpose tasks)
10. REBOL (language-oriented programming)
Additional/alternate picks: Haskell, Pascal, C++