I might have to say Suicune, but I love the lion-like appearance of Entei, and there's definitely something about Raikou that draws me to him. It's a pretty tight race, but I may have to say Suicune. It's so imaginative and fantastic (as in "fantasy"), and majestic. It was so cool in the 4th movie. I remember really trying hard to capture it in my Silver game.