This is my first tutorial, so please read to the end before commenting or voting, so I make sure of that change or add, I'm Brazilian so forgive me the mistakes of grammar.

To begin the introduction Zed is by far a great solo top by the single fact that your first skill has a very big damage and has great range, not counting the shadow who rises the range so considerably and makes his escape from ganks much easier or even get out of a dive to kill someone.

Pros
Have a great amount of damage.
Can escape easy from ganks.
Have a great range.
Have no problem in solo against any opponent.

Cons
Energy ends too fast, so pay attention or you can end up dead.
The shadow cooldown it's insanely heavy in the early game.
It's a little complicated to learn how to slow and catch with his shadow.

If you are an experienced top laner, i recomend to you to start with dorans blade, the extra life, damage and life drain will be awesome in the early game, but to the ones who are starting now, i recomend to begin with boots and 3 potions, i only say this because he will deal a lot of damage from long distance and you will not have to worry with your life to much.

The next itens i put up there will maximize your damage without letting you vulnerable to any enemy, in place of GA you can build a force of nature if you're taking a lot of magic damage.

Youmuu's will give you the cooldown reduction you need, not to much and not to less, and will increase your attack and movement speed, you can build a phantom dancer in place of it but i prefer this one only because of the cooldown reduction and armor penetration.

You can do the next sequence to kill for sure an opponent
Q -> W -> E -> Q -> E
Because of his energy it's the farther you can go but with an blue buff and can add an q in the end of the sequence.
Q -> W -> E -> Q -> E -> Q

Marks of armor penetration will neutralize the possible armor of your enemies, he starts with a good amount of damage, so marks of damage are not SO necessary in the begin.

Second, armor and magic resist are the most important item in any solo top, they raise your defense power against any character you can face.

Last and most important, why not give just a little damage, 6.8 with te 15 of armor penetration will deal so much damage that you will be crazy in the early game, and will give you the chance of take a first blood.