With her experience in action, drama, and horror (as both victim and monster), Chloë Grace Moretz would be my top pick for Abby.
I've seen a lot of fan castings calling for Florence Pugh to play Abby, and I think she'd be a great fit. But hear me out: Pugh is just too damn charming.
That's not to say Moretz isn't charismatic, but Abby needs to have an intimidating edge to her that I feel Moretz's experience in horror would be better suited for.
Whoever HBO goes with for Abby has their work cut out for them. Divisiveness over the character aside, it's important to remember an advantage that video games have over live action: They can combine different faces, bodies, and voices into one character.
Ultimately, Abby was portrayed by three different people in the game: Laura Bailey (voice and movement), Jocelyn Mettler (face model), and Colleen Fotsch (body model). The live-action actor will have to embody all of those different elements into a single performance.