What is Ashe's build in League of Legends?

Ashe is an attack speed and an attack. So the best item you definitely should get is Phantom Dancer. That should be your second item. Your first item is Berserker's Greaves. Second is Phantom Dancer. Third is Infinity Edge. Fourth is Bloodthirster. Fifth is another Phantom Dancer. And finally, the sixth item should be another Infinity Edge.

Berserker's Greaves -> Phantom Dancer -> Infinity Edge -> Bloodthirster -> Phantom Dancer -> Banshee's Veil

ADDENDUM: I wish to make sure that you understand that there is not one way to build a champ. I personally prefer getiing the Infinity's Edge first (for more damage output) before I build a Phantom Dancer. Some games also call for other items, such as Quicksilver Sash for example.