For sound complexity, one language stands out. !Xóõ, spoken by just a few thousand, mostly in Botswana, has a blistering array of unusual sounds. Its vowels include plain, pharyngealised, strident and breathy, and they carry four tones. It has five basic clicks and 17 accompanying ones. The leading expert on the !Xóõ, Tony Traill, developed a lump on his larynx from learning to make their sounds. Further research showed that adult !Xóõ-speakers had the same lump (children had not developed it yet).I guess all those diacritical marks bruise the larynx when they bump by.
That's not the end of the article -- while the sounds are difficult, the sentence structure of !Xóõ apparently isn't.
I propose we cross the sounds of !Xóõ with the grammar, lexicology, syntax, and semantics of Tuyuca and create a language that most humans could never hope to speak, even if inculcated from birth.
PS: This is not a new observation, but Mark Twain wrote something clever about nearly every topic that matters.