I think everyone has one Ghibli movie that they love more than the others. For me, that film is Princess Mononoke. The animation and music are both superb and, although is gorier than all the rest of the Ghibli films combined, the story is second to none. It was my first experience of Studio Ghibli and moved me more than any of the rest. Though Nausicaä was also very touching. And had giant woodlice.
My other favourites include Spirited Away, Castle in the Sky, The Cat Returns and My Neighbour Totoro. In fact, the only ones that I really disliked was Pom Poko (though it did have the immortal line "They used their family jewels in a fierce kamikaze attack") and Whispers of the Heart (merely because it was long and very little happened).