I'd say Go Beyond is the strongest contender, followed by Wonder of U.

Firstly, GER's ability is called Return to Zero. It resets all thoughts/actions to their origin point, to zero.

1) Go Beyond
Go Beyond quite literally does not exist. Interestingly enough, the plant appraiser described it as "close to zero."

How can GER use RtZ on something that doesn't exist? How can the actions of Go Beyond return to their origin point if there is no point of origin?
Then again, Gappy has to willingly use Soft & Wet in the first place — and that's something that can be reset. Which leads me to my second contender

2) Wonder of U
WoU is an extremely long range and autonomous stand which operates completely independently of its user. Tooru has limited control over it, but never uses it for anything more than observation. For all intents and purposes, WoU is a steadfast bodyguard.

Its main ability, the calamity, is an observable natural/supernatural phenomenon in the P7/P8 universe not unlike Ball Breaker. Wonder of U's ability isn't the creation of calamity, but rather the manipulation of it*. As such, even Tooru himself says it's very unpredictable. He's aware of it, he takes advantage of it, but he can't control it and doesn't know when or how it will take effect. The only thing needed for Wonder of U to activate and begin targeting someone is the mere intention to pursue Tooru/Akefu/WoU itself. Even the thought will trigger it.

*unless WoU directly touches an object and fills it with calamitous energy, in which case RtZ would undo with ease

The question is — can RtZ counter this? Can it reset something which is activated automatically, without any direct thoughts/actions from the user, and manipulates what appears to be an immutable property of the universe itself?

Of course, this is just talking about abilities. Let's say it's a 1v1 physical fight without any abilities, just stands punching the shit out of each other. WoU would be pulp, as would S&W. In fact, very few people would be able to fight GE and come out on top, let alone GER. I'd say Jotaro would be the strongest contender then.