All of them for me, at one point or another. It shifts about from time to time, which one I consider my favourite. Spirited Away is usually my default choice for top pick -- though right at this moment I'd lean toward Laputa.
That said, I'll never tire of rewatching Porco Rosso.
(Whisper of the Heart never gets enough love, by the by.
Probably one of the best for emotional introspection, especially if you're a creative sort.)
When I weigh in the graphic novel along with the movie, Nausicaa is the sentimental favourite by far. Even if it technically isn't a Studio Ghibli movie -- for Laputa: Castle in the Sky was the first Ghibli movie -- Nausicaa provided the launch pad for the creation of the studio. So it can be considered an honourary Ghibli movie, even if that is not technically correct (the very best kind of correct!).
Nausicaa will always be my favorite Ghibli movie*, even at the times it isn't, simply because it felt like a watershed moment in animation at the time it was released due to the complexity of detail and the deep themes and well-written plot and complex characters. I'm glad that my introduction to anime back in the 80's included both this and Akira. (I did watch all the giant robot series on TV back in the late 70's, early 80's, but they were vastly different in terms of quality and thematic complexity)
Even better, like Akira, Nausicaa hasn't dated much. The attention to detail and movement has let it age better than other 80's anime releases like Galaxy Express 999, Macross (The Movie) and (hurhurhur) Transformers the Movie... though if you like animation, you owe it to yourself to watch them all at some point.
*Actually, Nausicaa is one of the reasons I never got on the Mononoke hype-train. While I recognize Mononoke is a great movie, I felt that it recycled too many themes and story arcs from Nausicaa... which felt broader in scope.
Another internet beer for you, good sir.
Nausicaa was also part of my introduction to anime -- one of my high school friends regularly had Japanese exchange students board at his house, and one of them gave to him a VHS with Tonari no Totoro, Nausicaa, and most (but not all D: ) of Laputa, taped from Japanese TV (thus no subs). I must have watched that tape a dozen times or more. (I think Kiki was on there as well.)
Re: the point about Mononoke and the recycling of themes: Miyazaki is prone to this throughout his works. I recently stumbled across a short graphic novel by Miyazaki called Shuna no Tabi (The Journey of Shuna) from '83 (which is when work on Nausicaa began, incidently). The themes and character designs resurface repeatedly in future Miyazaki works, most especially Nausicaa and Mononoke.
Unfortunately, The Journey of Shuna, as far as I know, never got an official English release -- the only copies I could find were the original Japanese publication.