No, it is not his numbers, it is not his kit, it iIS how he scales.
Rengar is a strong solo lane pick. He has a free sustain, resistence, and nuke with cc. He has a escape that hunt someone offguards.
He makes you fear the bush and get zoned by the bush.
No matter how deep you nerf him, he will still be a good pick. It is the same thing nerfing Alistar.
This is not a problem, it is his kit.
But now take a look at it. He scales so well with level. All his abilities reduce thier cd by the level. They have good base damage and they still scales well with items. Of course anybody builds AP on Rengar anymore. So builds items doesn't affect his W.
Now, look his other abilities: 70% bonus damage on his E with on-hit efffect and good base damage nuke with cc.
150% attack speed bonus with Savagery with his Q.
43 armor and magic resist on his W, still doing damage and healing with Savagery.
Mov. speed bonus plus stealth on his R.
By looking this, I see: Great Burst and DPS with his Q; Sustained Damage, tankyness and sustain with W; 'okay" nuke with CC; Escape/initiation with R. Looking to this kit, you can already say: "This guy is OP, because of this kit"
This kit can make him OP, no doubt. Although, what makes him OP is the quantity of bonus attack speed he gets from his Q and the tankyness by the level he gets.
He already has a good armor/ level, attack speed/ level, why does he needs this amount of stats? I admit he has problems with HP, but I think his resistences make him tanky enough so he doesn't need to spend gold in HP items.
Seriously, he is god-tier in lane, god-tier 1vs1, strong mid-game and good late-game.
Saintvicious, Ocelote, Westrice, HotshotGG, Wingsofdeathx, 4notonebadbrad, hafu already said that he is OP right now.
"Oh but he only have 44% win rate on lolking." Alistar had 45% and kept getting nerfed. He has low winrate because he is always banned and when he is not, a person say "Oh, he is not banned, I am playing him and kickass!" But he is not used playing him and he fails.
Summarizing: He scales so well, his lane is good, his teamfight is not bad and his 1vs1 is god-tier. This makes him OP.
If you agree/disagree, you can post here to share your opinion.