[Count Belisarius]

I do not know about the younger generations of characters, but as far as some of the "adults" I do have a few thoughts:

Queen Cersei Baratheon = Demi Moore (with dyed hair and colored contact lenses)

King Robert Baratheon = John Goodman (big, fat, drunk and loud)

Ned Stark = Liam Neeson

Catlyn Stark = Cate Blanchett

Jaime Lannister = Brad Pitt (a perfect fit as previously mentioned)

Tywin Lannister = Patrick Stewart or Kelsey Grammer

Littlefinger = Edward Norton

Roose Bolton = Jeremy Irons

Jon Snow = Orlando Bloom

Barristan Selmy = Sean Connery (who else?)

Red Viper of Dorne = Benecio Del Toro or Pearce Brosnan

Sandor Clegane = Mickey Rourke

Beric Dondarrion = Clive Owen

The Mountain = Ralf Moeller