What's hardest you ask with a straight face. The beginning, middle, or ending of a story? Yes, I answer. Depending on the story. But since I suspect a single word answer would bring a stray torch, or possibly a pitchfork my way, I'll elaborate. A lot.
Beginning and middle.
Here's why. A story, for me, cannot start until I know how it must end. To know how a story must end, I have to understand the characters. I must know what motivates them, what drives them (these two things *sound* like repeats of one another - they aren't). Motivation is background work. Drive is right now work.
Acting Jargon Alert!
Drive in acting is actually called 'Objective'. 'What's your objective in this beat?' That's the question that gets asked about a distinct unit of action - the actor way of saying, "What are you trying to accomplish whilst you stand there yelling and waving a gun around?" The answer is supposed to be a transitive verb. You know. To cajole, to convince, to brutalize, to convert, to #add your active verb here#.
End Acting Jargon.
I must know what brought my main character(s) to a story. In Enemy Within - the heroine was motivated by desperation to prove she isn't broken. Her drive: to regain command (of a ship). Enemy Games - the main character craved identity (having been defined in other people's shadows all her life). Her drive: to cure a disease.
Once I know the motivation and the drive, I know how the story must end. Yeah, I'm not clear on how that happens, either. I just know that endings pop into existence. Like spontaneously generating goose barnacles.
The trade off is that I start a book six times. At least. And complain to anyone who will listen about how little progress I'm making while I write those various scenes. Thus, I say unto you, beginnings are hard. Until they aren't anymore.
Then the middle is. I wander. I dally. I dip my toes in the chilly waters of 'talking heads, not enough actual conflict'. I recycle some of the scenes I'd written for the beginning, but it turned out, were part of the middle. At about the 2/3 mark, I realize that the scene I had planned for the climax of the story is really the midpoint. If that. And then I flounder in the middle MORE because someone just redrew my roadmap. The GPS in my brain has the LONGEST 'recalculating' function...and for me, here lies the danger point. This is where, in the early days, stories littered the roadside, abandoned, because I couldn't seem to find a way out of wandering. It still happens. I'd like to think less frequently, but it does still happen. Usually, a revisit of the motivation and drive issues for all characters will give me the directional club between the eyes that sets me right again. Usually.
Still. Momentum begets momentum. Most stories, by the time I hit the 2/3 point, I can see the light at the end of the tunnel and I'm invested in getting my characters run over by that train (which should be a manifestation of their greatest fears based on the motivation and drive mentioned earlier - some days I'm better at that than others.) This is why endings are typically not a major problem. See me hedging my declaration? Totally superstitious and I do not intend to curse myself in that regard if I can help it. Having two parts of a story dogging me are enough.