1. Kiki's Delivery Service
It's a perfect film for all ages. Nothing too weird or overly melodramatic. Just a fun story with nice characters.
2. Castle in the Sky
It's an adventure movie with air pirates. Need I say more?
3. Princess Mononoke
It has one of the more interesting stories from the studio. It's a little on the dark side and it's far more violent than anything else from Studio Gibli. It would make a very good live action movie. Elk riding for the win!
Honorable Mentions:
Ponyo
Howel's Moving Castle
NausicaA of the Valley of the Wind
Porco Rosso
Related and Great:
The Castle of Cagliostro (Hayao Miyazaki but not Studio Gibli)