My favorite work from Studio Ghibi has to be Ponyo.  Hands down.  That movie is just so adorable, and hits you right in the feels as the story progresses.  Not to mention both Ponyo and Sousuke are just too adorable and pure for this world.  

But another close favorite of mine would have to be Howl's moving castle.  That movie was well done, had amazing characters, and I loved Sophie and Howl's relationship has the movie progressed!