Half-Life because both games were very advanced for their time, and included a lot of innovative and original gameplay elements, as well a story that was good and didn't get in the way of the game.
Halo, which I don't like either, was the first console FPS to offer such a solid MP experience and to have very easy to handle controls.
Harry Potter because they're great books, which are perfectly paced, don't shy away from vocabulary while not being pretentious about it, because, like Tolkien, Rowling uses old folk myths and legends and adapts them to her own world. Also, they created a universe that is fantastic in theme but believable in its execution.