I've picked Zed up at the start of season 3, because people said his ultimate was "weird" and "not comprehensive"
So I went on to take a deep look into this guy Zed

He is an amazing champion, I found him very easy to win lane
The killing starts when I hit level 4 in almost every game.(including ranked games)

I won't say Zed is over-powered, he has weaknesses, which he cannot lane against tanky or high sustain champions like Singed Malphite Zac
And out of all the countering champions, I REALLY HATE PLAYING AGAINST Kayle
Because her ultimate Intervention save whoever i cast Death Mark on

But overall, I enjoy the craze that Zed gives me when he kills a champion

Zed is an assassin who is able to kill AD/AP Carry in 3 seconds

Explanation

For most team composition, Top laner and jungler have to build tanky, so that they can absorb damage for their team.

That means if Sad goes top, your team would lack a tank, usually a team has 2.
However one tank may work with a well planned team composition, but for most cases, its not recommended.

Tanky Zed is useless, because Sad without AD cannot assassinate the backline of the enemy team, while he is not a material to be a tank, considering his passive and skills.

Perfect tank must have either hard crowd control skill e.g. Curse of the Sad Mummyor defensive skill e.g. Granite Shield
Zed has none of them.

So, Zed can only played in one way ---- Build AD and Kill

High Killing power

Poke with Living Shadow plus Shadow Slash or Razor Shuriken

Strong passive Contempt for the Weak (which cut the last breath from them)

3 Blinking Skills Flash Living Shadow Death Mark

Scale well from early game to late game because of Contempt for the Weak


Very squishy as Zed has no defensive skills

No natural sustain in lane

Don't know when is the right time to dive

Need to position very well in teamfights

Red flat Armor Penetration x9

For all champions, they have at least 9+ armor (lowest starting armor is Sona) at the start of the game excluding runes
Penetration is important because it makes your damage to deal nearer to 100%
With zero penetration, no matter how much AD you have, it is useless.

Yellow flat armor x 9

To negate the basic attack damage from ranged champions
Most AP mid champions are ranged

Blue Magic Resist Per Level x9

Mages don't do heavy damage early, so magic resist per level makes you tankier to survive the later higher damage from mages

Quints AD x9

Better Last Hitting
More penetration may be wasted as your enemy may not have the enough armor to be penetrated

Note
-100 armor makes all incoming physical damage to deal 1/2
-50 armor makes all incoming physical damage to deal 3/4
-Same for magic resist

ALWAYS ALWAYS ALWAYS
Build The Brutalizer first
It gives armor penetration, cooldown reduction and AD

Last Whisper
When your enemy is stacking armor heavily

Bloodthirster
Increase basic attack damage and all skills damage dramatically