For me, it would be nim. A single language that can run on all sidesof the stack, great to write in and ~fast compile times. It helps that I have been toying with it for years, but it's definitely the language I enjoy the most. Of course the ecosystem is not huge, so you have to roll your own solution a lot, although it's getting slowly better.

Jester+custom HTML dsl+compiling nim to js makes for a good experience, imho