The Economist has an absolutely fascinating article about difficult languages worldwide. I won’t give away which one is the hardest, but here’s an interesting one:
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).
Difficult Languages: Tongue Twisters [The Economist]