Zed's game is to poke and harass from long range and then make use of his multiplicative scaling when the time is right. Sure it looks like he has low ratios, but I've exploded tanks pretty handily with Zed in the mid to late game. See, the trick is that his W potentially Multiplies by 150% the scaling of his Q and E, and his ult potentially Multiplies by 200% and then adds on another 50% damage a few seconds later. Add onto that that if you come anywhere near him in the late game with <1/2 hp he's going to instagib you with a 400+ damage AA + 10%max HP and you have a really nasty finisher in fights.
Oh, did I forget the other thing his ult does? Yeah, it's a trollult, meaning you can use it to dodge that most meaningful combo ability and then rape your defenseless victim right in the butt. It provides untargetability, which is a pretty rare stat in this game.
I should point out that Zed has two to three potential blinks in his kit if you use him correctly, and, as such, has insanely high mobility in combat, and straight up unreal juke potential anywhere near brush.
Yes, Zed is pretty hard to use to full potential, but, if you get it right...
Well, let's just say I've 1v1 murdered carrying mid game Lee Sins while I was in unfed status, I've asploded plenty of Nunus before their ults could go off, Malphite is pretty lulzy, especially with black cleaver, and Jax ain't dodging nothing including the killing blow. I can't say I've had much experience with Udyr, and Xin Zhao can go either way, Riven can give you a bad day if you fail to burst her, but other than Riven, you'll probably get away. Elise is pretty even with the long ranged harass, but TBH you're going to be doing a lot more damage and have a lot more escape capability when it comes down to the melee, just don't get stunned. Vlad doesn't have anything, he's weakest late game where Zed is being the harassmaster ninja, and even later on his ult is insanely vulnerable to well timed dodging. As for Teemo, he can be annoying, but, like Malphite, Zed can outrange him, and even MORE than Malphite, Zed needs no AAs to kill a Teemo.