I like the silhouette of the dragon on the top left (bottom thumbnails), but maybe with the eyes of the one on the bottom right. Is he supposed to be funny? Scary? Smart? Those eyes look a little goofy, but if you keep that mouth shape and a large brow it may give him the 'serious and seen-all' wisdom. The one on the top left also feels like he may be older in comparison to some of the others.
The one on the top left is my favorite too, but I was worried that he was too generic. He's supposed to be a little on the goofy side, but also scary at first until they find out that he's just grumpy because he needs a cough drop after all that fire-blowing. Thanks for your feedback!