Personally? Lua and/or moonscript. Not only does it transpile to javascript, you can even compile the Lua C code to Web ASM and run it in the browser directly with relatively good speed. Aside from that, you can use it to program games (löve and many other frameworks), http backends (openresty, luasocket, luahttp, etc.), using LuaJIT you can even write quite performant code for computationally expensive processes. Aside from that, if you write applications from scratch in a language like C, C++ or even Pascal, you can easily embed Lua as a scripting language.
But again, that's just my personal opinion, and I must admit that I am somewhat of a Lua fanboy.
