According to IMDB, Stannis Baratheon in Game of Thrones is being played by Gerard Butler.
Updated June 2011: Looking at his recent photos, Gerard Butler would be a suitable Stannis. Whether this is true or not, we'll find out soon enough. It has been mentioned that this is a hoax and HBO has not confirmed whether Gerard is the man.
Gerard Butler is known for his roles in 300 and Rock 'n' Rolla. I'm a bit of a fan so I'm excited to see if he is the official Stannis Baratheon.
Back then, I thought Rufus Sewell would be a good Stannis. Another actor I've thought of since then who would be a good Stannis is Mark Strong, who was also in Rock 'n' Rolla and was the villain in the recent Sherlock Holmes.
Stannis is Lord of Dragonstone and Master of Ships on Robert's Small Council. Stannis is known as a brooding, humorless man known for harsh and unyielding sense of justice. His stubbornness and determination are legendary, most famously shown in resisting the Siege of Storm's End under desperate conditions. He is also a capable naval commander, having trapped and destroyed much of the Iron Fleet during Balon's Rebellion.
His wife is Lady Selyse of House Florent, a mighty noble house sworn to Highgarden and House Tyrell. They have one daughter, Shireen, a small girl of poor health, and bearing a disfigurement from a childhood ailment.
In the book series A Song of Ice and Fire, Stannis Baratheon is bald with a beard and a hard face - I imagined him to have a villainous look about him and Rufus Sewell was the one I visualized as Stannis, even though he had a lot of hair from the movie I remembered him from, which was A Knight's Tale.