So Rengar can 100-0 me, is there anything I can do about it?
No. Even though I have a Zhonya's, he will kill me from full health before I can use it. No other assassin can do that - Rengar is broken.
I don't understand how this fits into League of Legends' design values of Counterplay and Meaningful Choice. There's no counterplay for the squishy player unless you have superhuman reflexes. There's no reason to pick any other assassin over Rengar. Rengar in his current state is just as offending as Season One Evelynn when it comes to broken, except true vision is more difficult to get now. His play-pattern is similar to LeBlanc's, but he has faster damage output. And while LeBlanc is next on the nerf table, I can't find any indication that Rengar will be, too.
Let me elaborate: The victim has not even a second to react to a max armor penetration Rengar on Thrill of the Hunt. He will have a whole 12 seconds to find you (and he will have vision of nearby champions). He will kill you, and there's nothing you can do. He doesn't need to wait for you to blow Zhonya's or escape spells; he will kill you before you can react.
All assassins have high damage, but some deal it out all at once, and others do it considerably more slowly. Also, all assassins have some kind of gap closing ability, but some are more predictable than others. Most assassins do some kind of trade off. Assassins that have all-at-once damage, but predictable gap closing ability include Talon, Zed, LeBlanc, and Fizz. Assassins that have great gap closing ability but slower damage output include Akali, Evelynn Nocturne, Shaco.
Rengar makes no such tradeoff. He has all-at-once damage at perhaps the fastest output in the game, and he has ample time and cover to strike. He ultimate makes it extremely difficult to predict where or who he will strike. He has what every assassin covets but lacks: the ability to instantly and reliably kill a squishy champion. This is why he seems to outclass most other assassins, and why I think he should be nerfed. I'd love to hear a Rioter's thoughts on this.