While I've always really liked the design of Raikou, and found Entei to be pretty cool because of his personality in the third movie, Suicune will always be at top, simply because it was the only I've ever encountered and caught of the three. I met it randomly around Cherrygrove I believe, and got extremely excited, because one of my friends had told me about them before, and I hadn't believed him. Turned out they were real though. Obviously it fled, but I spent quite a while trying to catch it, and eventually got it. I've also always really preferred Suicune's design because of how majestic and beautiful it is. That seems to be a reoccurring theme with the ice/water legendaries :p