Firstly I like to take Flash and Teleport as my summoners because I always use tp on top as there isn't normally anyone to cover you lane if you are soloing. this limits the amount of CS you miss by a lot. The item build I found to be the most effective. mainly beacuse it will destroy squishy people with your ult late game and having last whisper allows you to hurt tanky people. the blade of the ruined king allows you to destroy common picks that stack HP like Garn Olaf and Mundo. With these 3 major damage items and your W passive you will easily have over 300 AD. this is by far enough to do you job of killing the Ad or Ap Carry. so that's why I build GA and Warmogs Armour so you can survive longer in team fights. For runes, any damage ones will do. I go scaling quints because they will give you more damage with your W passive than flat AD quints. Either AD or armour pen Marks are fine depending on what you have. i Made a commentary Zed here http://www.youtube.com/watch?v=zjihy-2_sjo

Basically late game Zed can kill anyone with his ult. in order to counter it make your self untargetable when the mark is on you. To this pick champions like Kayle, Fioria and Lissandra or make sure you have someone with a free flash like Ezreal Kat or Shaco. Then zed will not be able to attack you when you are marked so when it triggers you wont take as much damage. If you can not do this buy a Zhonya's Hourglass and activate t when he leaps to you.