Danish is said to be the hardest Scandinavian language to learn because of its speaking patterns. It is generally spoken more quickly and more softly than other Scandinavian languages. Danish is also flatter and more monotonous than English.
Foreigners complain that Danish is difficult to learn because same wordings can have differing meanings, depending on the intonation and context. Also, pronunciation does not necessarily follow spelling.
Source: http://www.everyculture.com/Cr-Ga/Denmark.html, https://matadornetwork.com/abroad/9-easy-languages-for-english-speakers-to-learn/