SQL is a great one because I've found so many devs either know nothing, or so bare minimum that it's easy to become the SQL guy on a team just because you're familiar with window functions. There's a lot of SQL out there too, and nothing has dethroned it after probably near 50 years now and I don't see anything coming close.

It's also one of the best bang-for-your-buck things to learn, time to usefulness wise. Whether you're writing JS, Java, C#, Python, Ruby, etc and have a backing DB - SQL is probably associated with the project. When it comes to NoSQL, there's a lot of great stuff out there for specific situations, but the querying is either fairly simple (with complexity on the application code or infra), or the query languages are very specific and less applicable to other software.

The core of SQL is very versatile and it'll be there whether you're at a three person startup or Wells Fargo.