C/C++
Now before you comment let me explain.
C/C++ is a harder language to learn yes, however it has aspects almost all languages build on. C++ being originally an extension of the C programming language, both revolutionized the industry. You wont learn C++ to code unless you REALLY like it in the end however C++ gives you the core fundamentals of what every programming language has (including functional languages, even though C++ is Object Oriented) it will make the gateway to any other language you want to learn. Go, Java, JavaScript, Clojure, Python all are a few examples o