All the best ones are short series or movies. I reccomend these ones especially, though they're all well known and you've probably seen them.
1.FLCL (Furri Kurri, or Fooly Cooly)
2.Blue Submarine No.6
3.Akira
4.Grave of the Fireflies
5.Rorouni Kenshin (This is more than 20 episodes)