Mew and Mewtwo, probably because of that first movie (and the sequel). The other legendaries are cool, but I just find Mew and Mewtwo's history really fascinating, and I loved their characters in the movies.
And the fact that Mew is so elusive in the games, like you really have to go out of your way to get one - even from the beginning it was mostly an event-only Pokemon, and so many people came up with ridiculous theories on how to "catch" it to no avail. Just makes it more interesting than the others to me.