So far, Arya has personally killed four people from her list: Walder Frey, Polliver, Meryn Trant, and Rorge.
Joffrey Baratheon and Tywin Lannister are also deceased.
Who will be next?
Once she makes it to Winterfell, Arya will likely try to direct some Stark forces toward Cersei. However, now that she has turned around to head North, it seems unlikely that this will be the very next person Arya kills.
Arya could be the "younger, more beautiful" woman who casts Cersei down, but we're betting on Jaime killing Cersei.
4. The Mountain (Gregor Clegane)
The Mountain is also on Arya's list. Assuming she makes it to King's Landing before killing someone else, and Cersei is still alive, Arya will be aimed at her like an arrow. If she's set on killing Cersei, she will probably have to go through The Mountain first, making him the next person she kills.
Does this count if he's already basically dead?
3. Ilyn Payne
It's about time the man who cut of Ned Stark's head comes back into the picture.
Melisandre is on Arya's list. Arya may head directly to Dragonstone to find and help Jon Snow. It is likely she will find Melisandre there.
He's not on her list, but he should be. He has used and manipulated her sister, Sansa, for years now. He even gave Sansa to Ramsay Bolton. Arya is going to be furious.
Littlefinger needs to die - everyone wants him to die - and Arya is headed straight for him.
I think we can all agree Arya isn't going to kill The Hound now, and Beric Dondarrion basically can't be killed. There's also Thoros of Myr, but who cares about him?