Could it be.. the oft forgotten about Poppy could be the hardest counter to Rengar?

Lets consider. While Rengar might have pretty great ratio's and damage and an auto attack passive that gives me a pounce, in order to truly be at his best he needs to use the bushes. Where are the bushes? At the edges of things. Why is poppy so difficult to play with effectiveness? Because her charge is nearly impossible to hit someone with. Granted this doesn't mean her charge isn't buggy (it is) but, basically Rengar hangs out against the walls of maps and Poppy likes to bash people into walls.

Does Poppy + Ward = hard rengar counter?

smart rengar gets sight seeing ward to kill poppy ward.

Counter to Rengar is having a brain. Same as Darius.

I think that Olaf is a great counter , his passive makes his stronger the lower health he is, his true damage is super strong and hard to trade successfully against. If you play passively when he has 4-5 stacks he is forced to heal if he does not want to waste them. I can send him packing or in the defensive with by throwing my Q right in front of me followed up by my e and throw my axe again which should be up if you managed to grab it. To wrap it all up, stay away from the bush, and only trade when you know you can win/ come out even.

Jayce.

counter to darius, rengar and jayce? all at once? in same team? couple of brains, janna, amumu, nunu, blitzkrank and jax.

janna to bait them then K-Airborne amumu to grip and snare al three, nunu flash in ult jax to finish the rest and blitz to pick up darius if he managed to be tanky enough to survive.

I've used Poppy, with flat armor yellow, blue, and quints, with AD reds. Spec'ed into the defensive tree for as much physical damage reduction as possible, with my remaining points in the offensive tree.

She is actually incredibly effective against Rengar when she obtains her philostone. Although she won't dominate or destroy this lane, she can definitely hold her ground and still get farm against him in lane.

I've done this physical damage reduction on Jarvan and Garen, They still don't win lane, but they definitely don't lose out either.