I can't choose a single one, but here's a list that I like a lot
海が聞こえる (Ocean Waves)
魔女の宅急便 (The one with the witch girl and cat) forgot the name
千と千尋の神隠し (Spirited Away)
ハウルの動く城 (Howl's castle)
Sorry If I got the english name wrong, I only watched the Japanese versions
Ocean waves is one of my favorite, but for some reason it is regarded as one of the worst ghibli movies ever.
Some people don't even consider it a movie because it is only 72 minutes long, they say "TV special" instead...