What's the most important thing of a story? The plot? The setting? The characters?

Characters are what make a story unique. Without characters, there would be no story. Without characters, setting wouldn't matter too much. Characters are what draws a reader in. People get connected to the characters, and then the characters matter a whole lot more than whatever the story is about.

It's difficult to come up with an "original" story nowadays, as most things have been done and there's only a few plots that truly exist. Sure, you can change the settings to make them more intriguing, but it's not going to do much in the end. The story is the same, and if the characters are the same, why would it matter too much if the setting changed?

However, if you change the characters, now you're going somewhere. Add a character who wouldn't act as any other character in that sort of plot would act. Add a character who's part of the LGBT community, has a mental illness, has a physical deformity. Break away from the basic stories by changing up your characters. You can easily make an original character by adding a few unique aspects in them found in hardly any other story.

The first thing I do when I'm plotting a new book is create the characters. I know they're definitely going to change as I go along, but that's okay. I'd much rather come up with engaging characters than an engaging plot. Yes, the plot certainly needs to be interesting, especially since that's (usually) what makes up the synopsis. However, once a reader gets enthralled with a story, they should be reading the story for the characters more than the plot or setting.

I also looked back to some of my old (trash) writings, and I am proud that--even though I messed up the plot badly--my characters were actually decent and some were outstandingly original. I found that I could actually bring myself to read through such horrible stories because of these captivating characters--and it's not just my own writing that I've found this to be true. 

I have read many books where the plot is so boring, so overused: The young character is destined through some sort of prophecy to go on some epic quest and save the world. Whatever. Half the time, if I see a synopsis like that, I won't even want to give the book a chance.

Yet sometimes I will find myself flipping through the pages and liking the characters. The characters make all the difference, make everything much more interesting, so long as they aren't the boring Mary Sue's/Gary Stu's that usually come with that overused plot. 

Your plot might not be good. You might execute the story badly. You'll probably make mistakes and wonder how to fix them up. Or maybe you messed up so badly, that the story just completely fell apart.

As long as your characters are intriguing and different from what we normally see in stories, then that shouldn't matter--too much. Of course, plot still holds a lot of weight when it comes to whether someone will enjoy the story or not. But if the characters are dry, then nobody will care about the plot and just close the book, since it's too boring to continue with if they don't connect with anyone.

I've had this happen multiple times, and with plots that sounded more original and well-thought-out. Even the settings seemed unique in some cases. However, the characters had little to no personality (or were so basic), and I ended up with unfinished books that I quickly got rid of. 

Characters are truly the core of the story. Make strong characters, and the rest will fall into place. Even if you mess up, there's still a good chance your story can be saved because of your characters. Let them lead the plot, and your story is in good hands.