What, only 8 options?
I own 15 studio Ghibli films... I'm Offended ;p
Personally, I can't decide.
It'd be The cat Returns, Princess Monoke, Laputa: Castle in the sky, Nausica of the valley of the winds, Porco Rosso, or even Kiki's delivery service.
Anyway, as a point of reference, here's a list of the ones that have been released so far (in the UK anyway - There's more in Japan)
(This is more or less in chronological order in which the films were originally made - The order of release in the west is somewhat different)
These films involve prominent members of studio Ghibli, but predate the studio itself being formed:
The little Norse Prince
Lupin 3
This one isn't technically studio ghibli either, but it is in essence the first film they did. (the studio formed the year after)
Nausica of the valley of the wind
Then the films so far released are:
Laputa: Castle in the sky
My neighbour totoro
Grave of the fireflies
Kiki's delivery service
Only yesterday
Porco Rosso
Ocean Waves
Pom Poko
Whisper of the Heart
Princess Mononoke
My Neighbours the Yamadas
Spirited Away
The Cat Returns
Howl's Moving Castle
Tales from Earthsea
Ponyo
So, yeah. 16 official films, 1 that might as well be, and two that are closely related to important people at the studio. XD