I'm now firmly into management but the one skill I use very regularly is SQL. By far the best investment I made in my entire career was a little bit of relational algebra, some casual study of DBMS internals and a lot of hands-on SQL. The quasi-standards have also made it the easiest transfer across specific DBs and their flavours over the years.

PSA: Hi kids, here's a dinosaur with yet more free advice: put the tiniest bit of effort into SQL early on and watch the compound interest add up.

That's just because it is what you are comfortable with.

Many developers will jump straight for ORMs when given the chance.