Important in what sense? Some candidates would seem to be:
historical significance
most currently running code
most significant currently running code
most new development
most significant new development
most likely representative of future directions in programming languages
Most important in the sense of the internet. What is the language that keeps the Internet running?
Almost certainly the C and assembly inside all those routers and switches.
C
None. If there was any such language, everybody would be using that language only, which is obviously not the case.
I don't think that follows at all. One could be most important on any of the dimensions that I listed in my comment, and still other languages might be used. Even if there was one overall best language - which is a different question again - it might not be best for every circumstance.
Lisp, although it can be argued that it is not a language but just the building block of all languages