As a teacher, I've been fortunate enough to have students from all over the world. And every mother tongue comes with its own English mispronunciations.
Latin American speakers often struggle with the /g/ phoneme. Words like "colleague" and "dog" can present real challenges for beginning students.
Russians, on the other hand, dislike the voiceless glottal fricative (or, less nerdily put, the "h" sound) so much that there is a certain wizard in Russian cinemas who goes by the name of "Garry Potter".
Where are you from and what word is the most difficult for you?
PS. Here's a nice list of English phonemes.