I managed to catch Little Norse Prince on Film 4 the other day and though I enjoyed it, it's really a rather forgettable film. The whole film is on YouTube if anyone wants to see it (split up into segments due to YouTube's 10 minute limit, however).
My favourite Ghibli is probably Princess Mononoke yet I think Grave of the Fireflies is probably the best film the studio has made. Thus, perhaps both are my favourites, though I don't think I could constantly re-watch Grave of the Fireflies as it is sheer heart breaking material.
I'm rather surprised to see only one vote for My Neighbour Totoro in the poll considering that is generally considered to be one of the best and most loved Ghibli's. However, I found it to be a disappointment personally. I also expected a lot more from Howl's Moving Castle, though I still enjoyed it overall.
If there's one Ghibli I would recommend, if only to spread the word in an attempt to increase its popularity, it would be Only Yesterday. It's one of Ghibli's most mature films and in many ways transcends the boundaries of animation in its sheer realism (of course, naturally it does dabble in fantastical elements). One senses that it could be filmed in live action, even if every shot from the animation was kept the same. I think it offers a very different experience to those offered from other Ghibli films and I for one believe it to be one of the most remarkable films I've seen in animé (granted, I'm no expert of the genre).