ES6 modules. Tooling can take care of generating UMD from that as a single source of truth and since it’s the language standard for years now it’s the best supported format in the ecosystem. At this point UMD/CommonJS/etc are historical artifacts only useful to legacy codebases and by now they’ve all adopted whatever ESM->legacy compilation pipeline they need.
ES6 modules. Tooling can take care of generating UMD from that as a single source of truth and since it’s the language standard for years now it’s the best supported format in the ecosystem. At this point UMD/CommonJS/etc are historical artifacts only useful to legacy codebases and by now they’ve all adopted whatever ESM->legacy compilation pipeline they need.