So many to choose from...
Owen Teale is rarely mentioned, but his performance as Thorne is spot-on.
Liam Cunningham is the perfect Davos, no matter who he is with.
Dillane has been very good, but I think that
His performance with Kerry Ingram in S5E4 was his best
Kit's had his good moments, highlights for me being when he hands Sam Longclaw and heads beyond the Wall, and his silent scene at Ygritte's funeral. And so far in S5 he has dramatically improved, especially opposite Dillane.
Iwan Rheon and Michael McElhatton are incredible, and I can't read the books without picturing them as Ramsay and Roose.
Alfie as Theon was very good in S2, and Reek in S4/5 is so different it's insane how flawless his performance is.
Sean Bean and Michelle Fairley were outstanding: my favourite moments with them include their final meeting in S1, the Red Wedding, and Ned at the council meeting with Robert.
Speaking of Robert, Mark Addy absolutely made season 1 for me. My second favourite being Harry Lloyd as Viserys.
Charles Dance: phenomenal. Nikolaj in the bath with Brienne: Phenomenal.
Lena at the end of Blackwater was really impressive, and more recently I enjoyed her humour with Nikolaj in S5: "It's a threat..."
Jerome Flynn is Bronn. There can be no other.
Pedro with Peter was one of my favourite scenes from S4
Iain Glen really shone in S4E8, shame that got kinda overshadowed by Oberyn and the Mountain
I was really looking forward to more Ian McElhinney this season, but I guess that's not gonna happen
If I had to pick one performance that was better than any other, I would have to say Rory McCann in the Hound's last scene. Of course, there were other moments he was very convincing ("No fire!") but this really sold it.
I can't wait for more great performances from people like Jonathan Pryce and Alexander Siddig, I just hope they have a lot of scenes.
Wow, I'm really not as pessimistic as I thought I was..
ETA: Oh, and how could I forget about Jack Gleeson!