Hard to choose, as I'm a huge Studio Ghibli fan.
The one that holds a special place in my heart, however, is Princess Mononoke. Not only is the art, setting and story pure imagination, it was the first Ghibli film I saw, and was what introduced me to the collection.
Following in second place are My Neighbor Totoro, Nausicaa of the Valley of the Wind and Spirited Away.