JavaScript
As we all know, JavaScript is the programming language of the web. And because it's the only real programming language that runs in the browser (aside from Web Assembly, but that's a different story), JavaScript has become the most popular language in the world. With the rise of the Web as a whole, it's natural that JS would assume this position - even if it might not objectively be the best programming language.
Before we go any deeper, let's take a quick look at the history behind JS. It was created in 1995 by Brendan Eich, with whom I once had a brief interaction on Twitter (and I'm still starstruck to this day). Brendan was working at Netscape, the leading browser of its time, and they wanted a language to enable interactive behaviour directly in the browser. And that's what they got: In only 10 days, JavaScript was created.
Funfact, JavaScript borrows some syntax from Java, but besides that, the languages have basically nothing in common. The name "Java" in "JavaScript" was simply used as a marketing trick to create a sense of familiarity with the new language. They're about as similar as "Car" is to "Carpet".
As you can imagine, a language created in 10 days is not perfect. There were lots of quirks, rushed decisions, and things that simply make no sense:
Despite that, JS quickly rose in popularity. And as JS became more popular, the ecosystem around it also grew. That's why, today, JS has a plethora of modern frameworks, libraries, tooling, tutorials, questions, and communities around it.
It's also why there's been several updates to improve the language in recent years, with ES6 adding amazing features like arrow functions, destructuring, template strings etc. And why JS can now not only be used in the browser, but also for back-end development, machine learning, app development and so much more.
There's obviously still things to dislike about JS, but in my opinion, the flexibility and ecosystem far outshine any other programming language. It's also the easiest language to get started with: there's no need for a compiler or a runtime. You don't even need a code editor. Just open the console in your browser and you can begin writing JS.
And since JS is designed to work with HTML and CSS, it's also the easiest way to get stuff actually happening on the screen. When I started learning Java years ago, I wrote programs that would run on my computer and output stuff in my terminal. That was pretty hard to get excited about.
It's a lot easier to get excited (and stay motivated) when a few lines of code can make big visual changes that you can easily play around with and put online for the entire world to see. And to this day, that's the biggest strength of JS if you ask me.
All of the above combined is why you can never go wrong learning JavaScript. It's huge, pays well, can be used for anything, and will be around for many years to come.