All of my favorites from the studio. The only one I would consider owning at this point is the one I have yet to see - Porcco Rosso. That, and possibly Princess Kaguya and When Marnie Was there.
But as far as the great classics that are available to watch on my TV, this might do it.
The best Ghibli films IMO are the ones that combine stunning animation, loveable/memorable characters, and the magical feeling that anything is possible. The Ghibli films that show you images you've never seen before - The giant insect creatures in Nausicaa, the floating castle Laputa in CITS, most of the creatures in Spirited away, The moving Castle in Howl's Moving Castle, Totoro and the cat bus in Totoro, and the Forest Spirit in Princess Mononoke - are my favorites. Of course, I also own films like Grave of the Fireflies and The Wind Rises for the emotional impact of the stories and their great messages about war and humanity. And Arrietty is in there because it has, bar none, the most lavish artwork in any Ghibli film to date as well as a great soundtrack and many wonderful characters. Animation at its best.