Depends if you want something that's marketable now, or more if you're asking what's interesting learning for the future. Django and Rails are still powerful in web development territory, but I don't find them particularly exciting anymore. I'm quite interested in seeing where Go and Rust evolve to provide developer-productivity and performance at the same time. Previously, we've had these kind of splits where we hav…
I've heard one recruiter specifically advise I learn Scala or Clojure, but then I love in Finland, where the JVM it's still even more king of the jungle than elsewhere thanks to decades of mobile engineering focus. I'll be taking up Scala, as well as F# and Haskell, in the new year, because I've increasingly realized that whether functional programming really it's the new OOP or not, it's certainly the specialty I mo…
I also think F# is a better bet, because it is a first class citizen in the .NET eco-system, whereas Scala and Clojure are third party languages on the JVM.
In the enterprise space this little difference plays a big role when choosing languages.