As of now, I'd say Javascript because it's where I started to develop abilities to conceptualize programming. Prior to that, I learned some HTML but it was more of a copy and memorize endeavor, with little understanding of what I was doing. I could see me possibly really getting into php and/or perhaps one of the C languages but I only have a "BASIC" understanding of those.
If you want to get into C I recommended development under Ubuntu, which comes with gcc(GNU C Compiler) preinstalled.
Ok, I found C kind of daunting initially. Coming from JS, the way variable types are represented(int, char, short, long, etc.) is more complicated compared to the way you can just declare a var name and assign it a data type in JS. I understand some basics, like that a decimal number has to he represented by a floating-point type but how do you decide on things like which floating-point type to use for a decimal or whether to use short or int for a number given a circunstance in which they both work? Does it just come down to preference or are there more nuanced memory allocation aspects that one should be aware of?
The difference is that short saves more memory than int. The difference between int and short are not really felt with today's memory abundance, but theoretically if you wanted to write a smaller program that used up less memory and didn't need int's memory allocation you would use short: https://www.tutorialspoint.com/cprogramming/c_data_types.htm
I'm professionally a React(JavaScript) and Java developer. Prior I was a C# developer, and before that a Python/PHP/JavaScript developer. For me it's a tie between JavaScript and Python. Both due to their extensibility. If I were designing a large scale application today, it'd be node UI -> python service layer -> GO business layer. I might replace GO with Java or C# because I know them much better. GO is quite cool. I don't think I'd ever take another job in PHP. Slow cumbersome and ugly.