Zac needs to be nerfed

His success rate correlates negatively with rank...

High damage? Zac doesn't have high damage. It's high for a jungle tank, but it's low compared to most junglers. And during ganks, his initial E is about half the damage he's going to do.

Where's the downside? His peel is close to worst-in-class. So if it goes from "we're diving their carries" to "we have to stop them diving our carries", then he loses a lot of his power.

Also, his E is super slow moving. This makes hitting mobile champs in ganks very hard, since they have so much time to react. It also makes preventing team fight engages from him super easy for any champ with a displacement; Alistar/Thresh/Janna/Vel'Koz have about twice the amount of time to react and prevent a Zac engage (with a basic ability) in comparison to Amumu, and they don't even have the option against Malphite/Gragas/Sejuani/Hecarim. And that's assuming they don't see him charging up, because that turns it into the easiest initiation in the game to react to.

Of course, he could still need to be nerfed is his numbers are too high. But his kit has room for lots of counterplay. And as Kaioko mentions, people at higher ranks are adequately able to take advantage of those opportunities.

he isnt even in top 10 of win rate for junglers right now...