Since Rengar is inspired by predator, then its easy to understand that Sion will counter him easy, since he is inspired by Arnold. If you seen the movie, then you know what Arnie did to predator.
Post your toughs on this topic.

Lol, get on da choppa!

Yeah as any champ who has a lot of sustain for that insane dmg output. It would be funny if Cho´Gath and Sion would be the best counters.

The best counter would be Paul Anderson since he totally rapéd the Predator (and Alien) franchise with the "Alien vs. Predator" movies.

No. Because Sion is not covered in mud.

Think best counter to Rengar would be Thornmail. Most of Rengar damage is autoattack based. W and E are not very high damage abilities. They do some, but majority is in autoattacks. That is unless the Rengar happens to be some crazy AP Rengar. By the way, AP Rengar potential burst is even higher than AD Rengar's. Leveling up and gearing to AP might be a slight problem though.

^Wrong, I watched PhantomL0rd play AP Rengar a while ago on stream like he does with every new champ with an AP scaling.

He said that there's no way in hell AP Rengar is gonna work. His W, the only AP scale, has too long CD and the heal is too weak to rely on.

Also, Rengar has crazy burst at lvl 1 because his regular and empowered abilities don't share the same cooldown. That means if you have 4 Ferocity stacks and attack someone with Q you can use it immediately for an empowered Q dealing over 50% health of the enemy in less than 1 second.

I did not state that AP Rengar was early strong. In fact i did state that gearing and leveling to the AP would be hard. He does however hit ridiculously hard when he has some AP + Lichbane with his Q + W combo then. His W happens to have 1.0 ratio. The thing is, the damage starts to really show only after some gearing. Add Deathfire Grasp to the gear setup and you might understand the burst potential.

Safer route for burst would however be AD + Crit setup. Squishy, but will also reach absolutely bats**t crazy numbers.