I would also argue that he is the most well designed pure assassin champion in the game.
I would classify the following champions as pure assassins, and I will give a reason/s why I feel they aren't designed as well as Zed. I'm not calling these champions unbalanced or OP. I'm just stating why I feel Zed is better designed, and should be rewarded for it.
-Akali
I don't need to go into detail on this one. Pretty universally agreed that she is one of the worst designed champs in the game. From the synergy of over the top spellvamp and heavy stealth to being un-kiteable due to R
-Evelynn
Constantly being nerfed to unviablility because of the lack of counter play. Her nearly infinite stealth allows her to pass by wards and sneak up on people a little too well. When she does ambush a person she has very strong chase and misleading escape. Chasing an Evelynn is very similar to chasing a singed. She can damage you with hatespikes by spamming it during her escape, and can turn on you with nearly unparalleled chase.
-Fizz
Massive aoe damage and one of the highest bursts in the game (Veigar exists). Extreme mobility, burst and high sustained damage. He is also one of the most difficult champions in the game to attack even in meelee range. He has no stealth, but does that really even matter with this champion?
-Kassadin
R R R R R R R R R R R R R R R R R
-Katarina
Oh dear god. The Queen of aoe assassination. When I hear the term "assassin" I picture someone quickly taking out a single target. This champion can assassinate an entire team. Her resets give her power in ways that it shouldn't. There are situations where 1 person can easily duel her but the presence of a squishier team mate can put the odds in her favor instead of the other team. It is really strange that there are situations in most games where she has a better chance of going 1v3+ than she does in a 1v1 against one of the same champions.
-LeBlanc
LeBitch
-Shaco
The King of deception and stealth. This champion laughs in the face of Riot's philosophy on clarity. His entire kit is based on destroying clarity.
-Talon
His single target assassination has the least counterplay attached to it of any other assassinate in the game. Not to mention his initiate and escape are even beyond Zed's That is pretty much it. Rengar used to be a close 2nd, but since his rework he can be viably played closer to a bruiser. This is why I'm leaving Rengar off this list.
Remember, I'm not saying I have a problem with these assassins. I'm merely showing how Zed is the most well designed champion in the role that has most of the worst designed champions in the game.
In the end, Zed is a single target assassin, without stealth, that has very well established counter play. He can be frustrating to play against, but that is because of his role. It isn't because his individual champion design.