It's a tossup between Entei and Suicune. Entei was the first one that I was introduced to from watching the 3rd pokemon movie so I warmed up to it before I could see the other two. I didn't see Suicune until I saw the 4th pokemon movie about a year or two later, but because it's a water type it ended up growing on me as well.
I like Raikou as well but I don't like it as much as the other two. Don't really know why, even though it's got a cool design and everything.