Interesting question, Greg.

Most learners worry about the 'th' sounds in English - in 'thin' and 'that' - but, in fact, these sounds aren't physically difficult to produce. Anyone can put their tongue between their teeth and blow.

In my experience, one of the hardest sounds for non-natives to master is one that appears easy - it's the 'u' sound in 'cup' and 'done'.

This precise sound exists in very few languages. Learners don't realise that the English 'u' is a difficult sound, because they hear and interpret  it as a similar vowel sound to one in their own language. Many non-natives produce a sound similar to an 'a' when they try to say 'cup', and struggle to distinguish between 'cap' and 'cup'. In fact, many non-natives, such as Italians, can live a lifetime in an English-speaking country and speak otherwise perfect near-native English, but still be unable to pronounce a 'u' correctly.

I find difficult to pronounce the word 'world' as well as differentiating words like 'head and had', 'said and sad', 'dead and dad', and so on. It took me some time to even notice that these words are pronounced differently, but even so I'm not sure if I am pronouncing them properly.