I would not think Dever was the hardest compared to any of the others. Aside I think chosehch some say was the hardest. But what does it mean Dever was very hard
1 Answer
Although I haven't seen it in a while, I believe Rav Chaim Kanievsky's Derech Sicha quotes a Midrash explaining that the plague of dever accompanied every other plague, making it an especially difficult one to handle. (See here where the author quotes the Orchas Chaim who suggests the same idea.)