For me right now it's Kotlin. I believe it picks a good balance of simplicity/readability vs expressibility/power.

In particular I find it's generics, data classes, infix operators and extension functions very useful for expressing business logic concisely.

These features let me write half (or less in many cases) the code I might need to in Go.

More importantly is Kotlin is much easier to refactor. Part of that is there is just less code to refactor, the other important piece is tooling and IDE providing awesome automatic refactoring tools.

OCaml. For example here are some types I wrote that exactly describe an HTTP cache-control response header in a way that makes it impossible to create incorrect values of the header: https://yawaramin.github.io/re-web/re-web/ReWeb/Header/Cache...

Clojure

Elixir, Erlang, and Clojure, to name a few.