For me its the R pronunciation in words like "respirar". Its almost list "Hespirar"

Right now, the words I find challenging include 'Brazil' (the final sounds of the word) and some nasal sounds because we don't have them in English: avaliação, and evolução. Still, practice makes perfect! = A prática leva a perfeição. 😉 I take lessons once a week with a lovely native Portuguese teacher from Brazil. She is very kind, patient and understanding. She said that she believes my level is around/approximately B1.