Well hello you thirsty **********ers, my name is TheTrophyHunter and i was Diamond in season6, i wont talk about playing rengar, jungling and such things, this isnt really the guide to Rengar, its more like a guide on how to build assassin Rengar, you're gonna learn all that stuff hard way or read some other guides, im here only to present my build. AND ye i dont really care about "dont" and "don't", deal with it.]
And yes, this is my first guide and English isnt my native language, mistakes will be made, and this guide wont look pretty i believe.

Pros:
Rengar is the coolest champ in the game
Oneshot was never this easy
You can drink the blood of anyone you want, tanks, squishies it doesnt matter, you cut through them like through the cheese(at least in the early game, **** late game)
Incredible early-mid game

Cons:
Rengar was never really viable, he was broken but there was no comp he was made for
Your booty will her penetrated by late game tanks
Your ult is ****py now, there is no thrill of the hunt in the thrill of the hunt

I personaly use only 1 rune page and that is:
x9 Lethality Marks
x9 Armor Seals(since Rengar doesnt get armor from his w going scaling health is kinda bad)
x6 Flat cdr Glyphs(5% instantly)
x3 Scaling cdr Glyphs(5% at lv18)
x3 Attack damage Quints

You can use just basic Ad page:
x9 Attack damage Marks
x9 Armor Seals
x9 Magic res. Glyphs
x3 Attack damage Quints

Dont have a lot to talk about...
(quick explanation, the things i havent explained i find obvious)

Ferocity:
Spell dmg because rengars q isnt an empowered basic anymore.
You can go feast for a better sustain like i do, that new thing for early ganks which i find ****ty for junglers or expose weakness which is pretty good imo.

Cunning:
You can go either savagery or the mvm speed thing, i personaly like savagery more for a faster clear.
Assassin or Runic affinity, hmm, well i like assassin more cuz im invading a lot, A LOT, but runic affinity is a pretty good too, it gives you a lot better sustain in jungle.
Greenfathers gift is OK, though i find myself getting ****ed in the *** in teamfights so i like dangerous game more, id rather live than deal those 3%.
Presicion=dmg
Thunderlord for more burst, stormraiders surge if you are planning to stomp em.(i find it better than thunderlord on better Rengar players, but its a total **** on newbies)

Rengar is an assassin, and as such he shouldnt build deadmans plate, spirit visage, omen and those *****-*** ****ty items.
We, Rengar players seek blood of our enemies, we dont lick our wounds, **** that, id rather die and bring someone with me than just be annoying and survive, Rengar isnt Poppy, hes a ferocious beast.
SO THATS WHY WE ARE GOING HAM!
Jungle item isnt so bad, but in all honesty i think that skipping it and getting youmuus is a better choice at least for assassin Rengar, plus, if you really want to buy one, get blue smite, i find red smite on the new Rengar meh...
Edge of night(**** you vayne item) is made to counter disengages like Jannas ult, Braums ult, or Alistars q, so the plan is to activate it before ult, ult, use youmuus before you jump, oneshot adc or apc, with E midair, Q when you land, wait a little bit to use W so you can heal a little bit of dmg they have dealt to you and then use Q if you need dmg to kill or W to escape.
IE cuz of ult.
Duskblade cuz you are going almost(runes arent full lethality) full lethality build.
Black cleaver if they are kinda tanky, and you have a tanky team, Deaths dance for sustain cuz it is the only way to make your q heal you...

Develop your own playstyle, im just a crazy *** thirsty for blood **********er who wants to destroy adcs all over the world. You shouldnt listen to me.

Just aim your bola at where your target will stand compared to you when you jump on it, let say that youre jumping on scared vayne, and you think that she will go to the right, then cast bola to your right while youre in the air, not where vayne will be, hope you understood, its simple :^)
Use w to counter lee sins kicks, vaynes e and such things, you can predict them, you just need expirience
There is a thing possible with new rengar(and were a lot easier to do with the old one but oh well), and that is to jump to baron or dragon from the bush, heres the video of priest showing it, ive learned it from this Play Video
The current Rengar is less skillful champ than the old Rengar, but i believe you gotta think more now, well the problem i have with him is that im using my empowered q in 45% of cases, w in 50% and e in about 5%, it is really useless compared to q and w

Hope you **** some *****es up!