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.