I don't think it's Renly's crown, because his is made up almost entirely of stag antlers, whilst this one only appears to have a fear antlers. I'm betting on Stannis' crown!
I really like it though. It's simple and effective. I love the touch of battle silhouettes you can see in the crown itself. I myself would have liked to have maybe seen the faces of the five Kings in the Crown, faded in.
Wonder if this will be the final official poster like last years Ned Stark on the Iron Throne poster.
UPDATE:Just realised that it's actually Robert Baratheon's crown. So, Joffrey's crown now!
Oh and... Hi Res and Uncropped Official Poster:
Can't wait for the full size version of this poster to go on sale. If this is the final official poster, it'll look great next to my Final Season 1 poster with Eddard on the Iron Throne, nice and framed on the wall!