Because I always get asked this question and then argued with…

Consonants are more complex. Some (p, t, k, m and n are common) appear in most languages, but consonants can come in a blizzard of varieties known as egressive (air coming from the nose or mouth), ingressive (air coming back in the nose and mouth), ejective (air expelled from the mouth while the breath is blocked by the glottis), pharyngealised (the pharynx constricted), palatised (the tongue raised toward the palate) and more. And languages with hard-to-pronounce consonants cluster in families. Languages in East Asia tend to have tonal vowels, those of the north-eastern Caucasus are known for consonantal complexity: Ubykh has 78 consonant sounds. Austronesian languages, by contrast, may have the simplest sounds of any language family.

Perhaps the most exotic sounds are clicks—technically “non-pulmonic” consonants that do not use the airstream from the lungs for their articulation. The best-known click languages are in southern Africa. Xhosa, widely spoken in South Africa, is known for its clicks. The first sound of the language’s name is similar to the click that English-speakers use to urge on a horse.

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).

via Difficult languages: Tongue twisters | The Economist.