Zamenhof's Esperanto - it always felt a bit too, well, earnest in its aspirations to me. If we're talking about universal communication systems, then my favour

Zamenhof's Esperanto - it always felt a bit too, well, earnest in its aspirations to me. If we're talking about universal communication systems, then my favourite will always be SolReSol - https://en.wikipedia.org/wiki/Solresol https://en.wikipedia.org/wiki/Solresol [Note: I don't speak a word of either conlang]