If I had to choose one, then Book 1. One main reason is that the rest of the story is still to come.
But I also think that it includes some of Tolkien's most beautiful writing. The landscapes are so vivid, they resonate with earth magic; The Old Forest, the Withywindle, and of course, The Barrow Downs. When I read of these places it makes me want to get out and go for a walk in the woodlands and have adventures visiting ancient barrows and stone circles. And there are all those Hobbits of course.