I like Suicune. Its design is very sleek, and Ice types are my favourite.
In addition, since Crystal was the first version I ever played, Suicune was the first Pokémon I knew about. Of course I had heard about and seen pictures of Pikachu and Meowth, but overall I was generally clueless up until about five years ago.
Entei because of Pokemon Movie 3000.
Suicune a close second because of Pokemon Crystal.