Plot, generally. But they go hand in hand.
Obviously, if I’m writing a book in a series that’s already established, I have a basic cast of characters to work with already. But when it comes to a new series or a short story that’s independent of a series, I tend to start with a scenario or general plot idea and build from there. Null Witch started from an idea for a witch born without magic who moved far away from home to get away from her family. That’s more about the character than the plot of the book, but it’s definitely the setup for the plot! That’s why I say sometimes they go hand in hand.
Once I have a general idea, the protagonist is the next thing I tackle, and then the rest of the story. Sometimes it’s hard to know where you’re going if you don’t know who is making the choices that will get you there. But I try not to go too in-depth when it comes to fleshing out my protagonist. I sketch out a basic idea of their age, sex, ethnicity, socioeconomic background, strengths & weaknesses and fill the rest in as I go.