Ok here's the deal. I have seen and tried a lot of Ashe builds. Some i found here, some i tried out myself. Still, i think this is the absolute build for the hero.

I play Ashe a lot. At least enough to be able to see the pro's and the con's the hero has. A lot of people say that Ashe is kinda useless cause you know, low hp. I don't believe that. The damage output, the endless slow , plus the Arrow , the best starer in the game in my opinion, make her a pretty good addition to the team.

Few words here. Runes Ashe requires for effective playing are more or less standar. Almost everybody get the same.

Armor penetration : Invaluabe. Ashe does a lot of harrasing at the beggining. Beeing able to do full damage is a must. Plus it helps a lot in late game for tanks.

Mana regen : Ashe has a really low mana pool and regen. Since it's not possible to play her with utility masteries, you're gonna need mana regen from the runes in order to spam your Volley ability.

Cooldowns : It's pretty neat to be able to use your ulty often. These runes plus the brutalizer will make your arrow cd at about 50 secs or so.

Nothing to say here. Since Ashe is carry, attack masteries is a must. Also, it's a good idea to have the regeneration from the utility tree. There is absolutelly no reason to spend points on defence. Ashe will die easily, that's a fact. If the enemy team focus on you , you're dead, whatever you may try.

Ghost for chasing with frozen shot and flash in order to be able to escape ganks. Some people play with ghost + teleport, but i think that's stupid. There is almost NO chance you can make it alive out of a gank without flash..

The 2 doran blades with be a great help at the beggining. At least until you make your first bloodthirst.
Boots of swiftness is essential for kiting/chasing with your frozen arrows.
The brutalizer is a really nice item for the beggining. Nice damage boost with cd reduction. And it's a good idea to upgrade it later to ghostblade.
Zeal is a pretty neat item for ashe. I don't make it for the critical but for the movement speed and attack speed. Upgrate it to phantom dancer asap and you'll be running with 443 speed when others have 370-400.
The 3 bloodthirsts may seem a little silly, but in fact they do a tremendous work. You can make it to +300 attack damage just from them , plus the leech they provide is really nice. If the enemy team has more than 1 tank, i usually go for 2 of those + madred's though.

By the way, NO i don't make banshee on Ashe. It's idiotic and makes no sence whatsoever. If the enemy attack you in team battles your are dead, simple as that.