Java. It's everywhere, and due to the JVM it won't fall victim of "being written for that exact hardware" problem, like previous generation stacks did. Also, very performant, safe, has an insanely large hiring pool (more than people living in my country), one of the biggest ecosystems (the other two in the top 3 are js and python, and I would argue that js is very frontend-oriented ecosystem and for backend tasks it's not as mature/stable, and python is large in the data science direction. For regular software, Java's may be the widest), very good tooling (observability, live debugging).

(Though curiously many of those systems still run in actual virtual machines emulating the whole mainframe and whatnot)