1) Porco Rosso
2) Laputa: Castle In The Sky
After those two, it starts getting hard to put numbers on them. I enjoyed Howl's Moving Castle and Princess Mononoké (also Nausicaa, but technically, it was created before Ghibli was founded). Grave of the Fireflies was an interesting watch, but it was pretty heavy and I don't think I will be seeing it again.
The slice of life elements in Kiki's Delivery Service and Whisper of the Heart were pretty enjoyable, though I am not the biggest fan of My Neighbor Totoro. Yeah, the creatures are fun, but the end result felt like a mixture of two tones that seemed to simply ignore each others existence most of the time. Also, I wasn't really impressed by The Cat Returns and never got why it had to be a tie-in with Whisper of the Heart, since the tone is completely different. Pom Poko on the other hand, I enjoyed not because it was good, but because it was so freakin' bizarre (the whole testicle thing). I mean, yeah, I get the lore behind it, but it's still weird to see it in an animated film for kids.
I thought Spirited Away had great animation, but the story was pretty awful. I feel pretty much the same about Ponyo. However, were those two still had some form of pacing and "kind of" enjoyable characters, Arrietty had a bad story without any of those elements.
I haven't seen Tales of Earthsea, Only Yesterday, From Up on Poppy Hill, Ocean Waves or My Neighbors The Yamadas.