GraalVM/GraalJS is one I am most impressed with. It 'just works' and I've been able to integrate it into my java web applications easily. I mostly use JS in java to run handlebars.js and test JS code library that we use both on the server and the client.

I've heard nothing but high praise for GraalJS, yeah. I wonder whether it'd make any sense inside mobile apps – not due to any feature in mind, but simply out of interest because that's more the area I'm using JavaScript. The first-class interop with Java would be great for Android, though I wonder how cheap its interop with iOS (C-family languages) would be.