Bart D. Ehrman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 123.
Sometimes Mark used a Greek style of writing that is somewhat awkward or not aesthetically pleasing, sometimes he uses unusual words phrases, and sometimes he presents difficult ideas. In many instances, however, these problems are not found when Matthew or Luke narrates the same stories. This difference suggests that Mark was the earliest of the three to be written. That is to say, it would be difficult to understand why Mark would introduce awkward grammar or a strange word or a difficult idea into a passage that originally posed no problem, but it is easy to see why Matthew or Luke might have wanted to eliminate such problems. It is more likely, therefore, that Mark was first and that it was later modified by one or both of the other authors….