AssemblyScript is one of the best options, because if the only thing you care about is some high level language that outputs WebAssembly, then you are still using Web related tools.

Then D is quite easy to use (LDC alongside betterC mode, runtime is still not fully ported).

Followed by TinyGo, Rust, C#, Nim.

I would leave Emscripten, C and C++, only for porting existing code into the Web, as even with the sandbox memory corruptions of data allocated in the linear memory blocks are susceptible to occur.