+4
A lot of folks on the show are killing it. Charles Dance (Tywin Lannister) and Alfie Allen (Theon Greyjoy) come to mind, as does Conleth Hill (Lord Varys), though we've sadly seen precious little of Varys lately.
I also really like Natalie Dormer (Margaery Tyrell), not because she's particularly close to the Margaery in the books (she isn't), but because her performance feels wily and natural. Obviously, Michelle Fairley (Catelyn Stark) hit some high points recently and at other times throughout the series. A bunch of the lesser performances are great, too: Walder Frey, The Hound, Beric Dondarrion and Thoros of Myr, the Blackfish.
Everybody talks about Peter Dinklage (Tyrion Lannister), and he's great, but I fear we've seen all the range he can give us. He's got great mannerisms and delivery, but he doesn't handle real emotion very well. Lena Headey (Cersei Lannister) is passable, and probably perfectly cast, but she's not a great actress. Jack Gleeson isn't a great actor, but somehow he makes for a great Joffrey. (It really helps that he naturally looks like an evil punk.)
Others seem to be sort of on autopilot because their characters haven't yet turned a corner. Iain Glen is a fantastic actor, but he's kind of treading water: Jorah Mormont hasn't really changed much since Season 1. Kit Harrington (Jon Snow), as I've said, feels this way too.
The one who's hitting it out of the park for me, though, is Nikolaj Coster-Waldau as Jaime Lannister. He was a bit one-dimensional that first season, but over the second and third seasons, he's gone through much trial and change, and that will only continue. Not only does Nikolaj have Jaime pinned, but he's shown that he can display that deeply emotional and conflicted center that Jaime very definitely has.
Oh, and Kristian Nairn (Hodor). Totally nailing it.