That's actually pretty tough. Lena Heady is really good, probably my favorite of the living characters. She gives Cersei a lot of dimension and manages to even make her sympathetic at times despite being so hateable. And she's gone through a lot of different emotional states over the years.
Alfie Allen has done an incredible job with Theon/Reek, and I feel like he's maybe a bit underrated. They haven't given him much screen time at all this season.
Peter Dinklage is excellent, even if Tyrion has felt a bit one-dimensional in the last couple seasons. He did great during the trial.
A lot of the secondary cast is also excellent. Rory McCann, John Bradley-West, and Jerome Flynn play those characters so perfectly.
Pretty much everyone else is at least solid. Sophie Turner and Isaac Hempstead-Wright can be a bit iffy at times, now that they're playing "adults." They're not always convincing. I feel like Maisie Willaims has been a bit weak the last couple seasons, but that's more a matter of the material than her performance, I think.