This is a Full-AD-Rengar guide. Rengar is one of the best assassines in the game and he is pretty op with this build, I´ll explain why to play him like this and why this is so strong.

I play Rengar Top, because i think he´s not the best jungler, because you´re pretty useless with your passive against wards. You can gank other lanes with your ult too if you are Top, so just pick ignite and flash , this are the best summoner skills.

Why this items? Because Rengar deals pretty much damage. You dont need tanki items, because the best thing about rengar is your ult. Your ult gives you free-focus. You can just run into the enemy team and you can attack who you want, and no one can do something against you. If your Ferocity is full, you attack the adc with a double Q. With this build you kill him just with double q and maybe w or e or just an auto hit. And this is what you HAVE TO do. You attack the adc. Even if someone else in their team has low life. You attack the adc, because you deal so much damage, it would be just a waste to attack him. With your Q you get very much attack speed, so you just need some life steal ( bloodthirster and hydra) and you have very hard lifesteal. This will help you after you´ve killed their adc in a teamfight. I really had more Pentakills with Rengar than with every other champ. Buy Tiamat,"the Rengar-Item" early.

AD Masteries and AD Runes. Thats it.

Rengar isnt the best early game champ. You have no real chance against tanks or fighters until lvl 3, so just throw your e and keep running if you are attacked. Than, if you´re lvl 3, you can decide what you wanna do. You can keep playing save, using w to farm, or you can change to play aggressive. You´ll need your Ferocity to win a 1vs1, so try to get 4 Ferocity, than you throw your e and attack with your double Q. Or you can walk into the bushes to jump, but this will not help often, because the other top laner can just go away from the bush. What i prefer in most games is the "come-and-i-kill-you-tactic". Just get your 5 Ferocity and farm. If the other one tries to attack you, you INSTANT double q him , throw e and use w , activate Ult and jump again. In 90% of the games the other one is dead now.
But what to do , if the jungler comes?
This is what I like to do in most games: I walk into the Bush( with my 5 Ferocity) and jump to the jungler or the top laner, jump to the squishiest of them. Use your Combo, than you go away , use ult and jump again. This is your free kill. Even if you think you have no chance, try it! but if you are pretty sure, for example if the jungler is Dr.Mundo and the top laner is Shen, that this tactic wouldn´t help, just go into the bush and use ult.
And never forget to gank other lanes with your ult, you need the kills for your Rengar item. and kills on Rengar item means you get more kills.
How to play against counters, for example Lee Sin or Teemo?
Lee sin is your counter because he can see you with his e, going into bushes will not help and he can escape you easily. BUT of course its not impossible to win. Be sure that he cant jump back , throw bola , double q and you can get him. Against Teemo, use your w to heal, farm with your e, use your ult to get him if your jungler ganks.