It seems no one writes software in C anymore, although it most probably won’t win. Anyways, for the sake of completeness:
int main(){while 1 printf("1\n");)
You will get a warning from the compiler because you don’t #include <stdio.h>, but it works.
Another solution is a good old .com file for DOS. These are executed as is, pure machine code. That would probably win as it is actually executable without any fancy interpreters. I guess it would take about 10 bytes or so.