Definitely Princess Mononoke. It's like the LOTR of anime. It's epic on a mythical scale, it has a great story that's built on a morally complex scenario, very likable characters, and a deep, rich atmosphere that's darker, more dramatic and serious than your typical Studio Ghibli film. It's the kind of film that when I watch it I'm perplexed that a film so perfect can even exist. It's a shame that there's only one film to consume, I'd love to see a sequel series.
Lupin the 3rd: The Castle of Cagliostro
Although I do also enjoy Monster Princess and Porco Rocco. Most of them are pretty good in general.
I've not actually seen Arietty nor Howl's. I probably should. I think I've seen most of Ghibli's stuff otherwise.