I've only seen 4, can't make a decision so quickly :) Though Howl's Moving Castle has a special place in my heart, and even my mother loved Calcifer. I liked the book too, but should have read first because I couldn't forgive it for not being exactly the same, haha.
Spirited Away is 2nd.
Grave of the Fireflies is a hard to swallow 3rd.
I think I would like a lot more than Ponyo. I tried to watch My Neighbor Totoro but it wouldn't load :'c
Princess Mononoke was interesting but the character relations felt somewhat forced, Howl's Moving Castle didn't really make any sense (especially the end! Main protag girl's actions made no sense, and neither did going through the black door and seeing all that trippy crap)
But Spirited Away was well-designed and cute but still interesting with its own hidden messages, so I'm going with that.
P. S. Have not seen Totoro
Also haven't seen the newer movies like The Wind Rises even though I've seen it has really high scores and reviews
The Cat Returns, Spirited Away, Mononoke and Howl's are the ones I personally got in collection, but yeah they are incredibly expensive. I have seen and liked most of Ghibli movies until around when Arrietty came around.
The only ones I didn't particularly like is the Yamadas and that one movie about the woman in the countryside because I don't need that kind of drama in my life.