What is considered to be the most difficult sound for native English speakers to pronounce?
Not sure about research

From experience the sounds that people have the most issues with are:


the "r" sound when used at the beginning of a word. Best examples are 热 (re) and 日 (ri).
differentiating between zh &amp; j, sh &amp; x, ch &amp; q.
the ü sound as in 魚 fish (I think that is the one you mentioned).