Rank the ones you've seen. This can include anything Ghibli, not just Miyazaki.

For me:

1) Grave of the Fireflies
2) Princess Mononoke
3) My Neighbor Totoro
4) Kiki's Delivery Service
5) Laputa: Castle in the Sky
6) The Secret World of Arrietty
7) Spirited Away
8) Tales from Earthsea
9) Howl's Moving Castle

Those are the ones I've seen thus far. Although it doesn't count, as it wasn't made by Studio Ghibli, Castle of Cagliostro is an excellent film and should be an honorable mention.

didn't see all of them yet, but quite a few
the best one (at least for me) is without a doubt Mononoke Hime

it is also my favorite anime... and together with "The Lion King" my favorite movie of all time.

HOWL'S MOVING CASTLE! Love the book, the movie is even better.

Haven't seen more than a few of them (tragically) but my favorite is easily Spirited Away. It is just...perfect, or as close as it gets.

Top ten lists bore me terribly, so I'll just respond to the title:

My favorite Studio Ghibli film (of those I've seen) is the Secret World of Arriety

1) My Neighbour Totoro
2) Nausicaä of the Valley of the Wind
3) Grave of the Fireflies
4) Kiki's Delivery Service
5) Spirited Away
6) Princess Mononoke
7) The Secret World of Arrietty
8) Howl's Moving Castle
9) Laputa: Castle in the Sky
10) Ponyo on a Cliff
11) Tales from Earthsea

Grave of the Fireflies (easily in my top 10 of all films, really)
Spirited Away
Princess Mononoke
Howl's Moving Castle
Laputa - Castle in the Sky
Whisper of the Heart
Kiki's Delivery Service
Only Yesterday
The Cat Returns
Ponyo (close call between this and Arrietty, I thoroughly enjoyed both)

I have to say, I haven't yet watched a Ghibli film I didn't like (although I've yet to watch Tales from Earthsea, which I'm told is pretty low on the list of Ghibli classics). I wasn't too hot about Nausicaa, but I was under a lot of stress at the time and didn't much enjoy a lot of things, so maybe I should give it another watch. And Totoro misses the list, which I know should be a crime - but whilst it's undoubtedly a brilliant film, its squarely aimed at the very young and I didn't personally get a huge amount of enjoyment out of it.

1. Spirited Away
2. Nausicaä of the Valley of the Wind
3. Tales from Earthsea
4. Howl's Moving Castle
5. Ponyo

I haven't seen nearly enough...

Totoro. Easily, effortlessly, Totoro. One of the most intelligent films I've ever seen.

Special mentions to Laputa, Nausicaa, Princess Mononoke, Kiki's Delivery Service, Pocco Rosso, Spirited Away, and Ni No Kuni (it's basically an interactive Ghibli film, so it totally counts)

Iv'e only seen 2 so my fave is Spirited Away
I also saw Totoro

It is interesting to compare/contrast Nausicaa with Princess Mononoke. Mononoke almost seems like a... response to Nausicaa, in a way. Older and less idealistic Miyazaki examining the same themes and ideas that younger and more idealistic Miyazaki did in that classic film. They're similar in a lot of ways. Mononoke, though, is quieter, more observant, and more cognizant of the nature of humanity and its limitations, especially with regard to the natural world around it. Compare the pretty plainly villainous Tolmekians, especially Princess Kushana, for instance, to the much more nuanced Lady Eboshi of Irontown. Or the messianic Nausicaa to the more grounded and contemplative Ashitaka.

Although I'm not generally a fan of professional film critics, I do think Ebert's review of Totoro taps into a bit of what makes that film so wonderful. And one of those wonderful aspects of the film is the way it respects the intelligence and imagination of the children it depicts.

Castle in the Sky is my favorite

1. Spirited Away
2. Nausicaa
3. Mononoke
4. Porco Rosso
5. Howl's Moving Castle

Also...

Nice Porco Rosso throwback

I've only seen Totoro and Porco Rosso, and out of those I prefer Porco Rosso. Totoro is probably the better film, but give me womanizing rogue soldier pigs over...just about anything, really, any day.

My top five in no particular order would be Princess Mononoke, Spirited Away, Totoro, Laputa and Grave of the Fireflies. Back when I first watched Princess Mononoke, I was used to the western animations like Disney, so watching that Ghibli Movie for the first time blew my mind when it came out. Still need to get Ni no kuni, but damn I have other games I've been nagged to get for this month alone.

I never saw Grave of the Fireflies, but I think I will now because of these favorites

I have only seen Spirited Away (first one I saw), Totoro, Laputa, Howl's Moving Castle (all three came together) and Ponyo (used blu ray).

The nice thing about these movies is that my dad is always reluctant to watch them at first but he ends up fascinated by them. Ponyo was a special case, he thought it would be a dumb cartoon about an ugly fish and ended loving the art and the characters.

As for my favorite, I think that would be either Spirited Away or Laputa. I just love the idea behind these movies.

1. Spirited Away
2. Castle in the Sky
3. My Neighbor Totoro
4. Howl's Moving Castle
5. Princess Mononoke

1) Howl's Moving Castle
2) Princess Mononoke
3) Spirited Away
4) Grave of the Fireflies
5) The Cat Returns
6) Totoro
7) Whispers of The Heart
8) Kiki's Delivery Service

Kiki's Delivery Service...and that's it.