What is considered to be the most difficult sound for native English speakers to pronounce? When I was first learning Chinese, I found the e4 - 饿 &amp; ü3/ü4 - 女/绿 sounds to be the most problematic.

Has there been any research into this? If so, what sounds are generally the most problematic sounds for an initially monolingual English speaker to learn when studying Chinese?
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).