C.
C is high level assembler and manipulating assembly is the foundation of computer science.
What C provides over assembly is...type safety. Why is type safety useful and important? Bad assembly.
Why is a compiler useful and how do you use a compiler to prevent bugs? C compilers make understandable and idiomatic assembly and strong types make it easy to prevent lots of defects that are caused by imprecision and ambiguity.
From an employment perspective, data structures and many algorithms are easiest to grok and express with pointers. Most of the major programming companies will ask whiteboard questions in interviews and many of those questions test for proficiency with pointers.
Much of the key open source systems works are implemented in...C.
The other languages may give you a temporary advantage but I think only C gives a coder the foundational work for a long and successful programming career.