C99. It can do everything I want, and while there are some tasks for which it's not the ideal language, I can write my own libraries to supplement the existing ones.
If I'm going to be stuck with only one language, I'd rather use a language where some things are harder than in other languages, rather than a language where other things are simply impossible.
Agreed. Not to mention that you can just write your own flavor of any language you choose (most are already written in C). Plus, you've got til the end of time, so the fact that it'll take a few years to write the first one shouldn't be a problem.
If I'm going to be stuck with only one language, I'd rather use a language where some things are harder than in other languages, rather than a language where other things are simply impossible.