As a recent computer science graduate with a B.S. who has been searching for software opportunities for the past eight or so months, I can comfortably say that having experience in some tech stack or specialty is one of, if not, the most vital factors in finding favorable work (with that said, I'm based in the U.S. and the job market may look different in your country). The other key factor is having connections and that requires networking.
The fact that you're thinking about this while you're still in school is great but it's important to acknowledge that circumstances change over time and it's difficult to predict what will be in demand in the future. So, focus on the present and excel in your studies. Not all employers are like this but some care about your grades and having good grades just increases your odds of getting a better job. While you're in school, get as much internship experience as you can. And, if this is offered at your school, try to specialize in a computer science concentration (i.e. data science, cyber security, etc.).
To my knowledge, there's no single tech stack that's known for crazy pay. Especially if you're specialized, you can get solid pay in any tech stack or skill area. Certain tech stacks are more commonly used for certain tasks/work types, which I'm sure you already know, so perhaps you can combine that knowledge with your passions to narrow your focus.
With that said and given my experience, here's my advice:
- perform well in school and maintain as high a GPA as possible,
- put time into learning skills (Microsoft Certifications are great for this),
- put those skills to work by building projects you enjoy,
- connect with folks in the industry during networking events,
- practice coding problems (Leetcode) since you'll eventually need to pass a technical interview, and
- apply apply apply (EARLY) to internships & jobs.
I recognize that you want to know what specific skill or teck stack to focus on but I'm hoping this answer gives you a more holistic perspective on some of the variables involved. Of course, my experience is limited and I'm still learning too so take everyone's input with a grain of salt and keep an open mind :)