With a title like that, I could put a lot in here. There are so many enemies of writers today that it's getting hard to keep track. And the list will differ depending on who you talk to. Some people might say that the industry is the strongest enemy to writers today, while others would swear self-publishing is the writer's worst adversary. Some might go as far as to claim the Oxford comma is the strongest enemy of the writer. But, in truth, there are many possible answers to this question. In my mind, the writer's strongest enemy isn't something external. It's themselves, or at least it should be.
A writer's greatest, strongest enemy is themselves. There's many different reasons behind this. It's the writer's job to keep themselves active and motivated to write, to avoid distractions and procrastination, and actually do good work when they get down to it. That's a lot of responsibility put on a single pair of shoulders, and many continue to fight against distractions and lack of motivation. If you can't get over those writing blocks in life, then your output and work overall will suffer.
Still, beyond even the motivational barriers in life, the writer is their own worst enemy for one simple fact: you should best your work every time. A great writer knows that they're always learning, always developing their skills and changing. And writers destined to grow a fan base know that your writing should always be going up. In publishing, especially independent publishing, you can't afford to bore your fan base, much less new readers, with a book that is sub-par compared to the last. Or maybe it's even part of a string of humdrum books. It can happen, but it shouldn't have to. Always be changing, growing, and developing your skills and your writing and your dedication will show in the finished work. If you don't, then it won't, and your readers will notice.
In short, the writer is their own worst enemy because they're the competition. A writer should always be pushing the boundaries of their skill and of their craft, allowing their style and work to grow. With every book a writer puts out, they should be letting out a small sigh and saying "damn, now I've gotta beat that." Past you grows stronger with every great story you add to their back list, but you've always got to stay ahead of them. Grow your writing enough, and your audience will grow with it.