This was a build that I ran several times during season 9 and 10. At first I just built like 3 IE because it was funny and you get huge damage autos. However the build has actually become quite viable in season 11 as most crit items now have AD and collector has lethality!

Rengar's passive can give him 25% extra AD once fully stacked (which isn't that hard) which allow you to get 500+ AD when fully built. With this criting and with IE, your autos start to do 1500+ damage. Now with your Q and the bush leap, you can easily do 4000 damage in about 1 second.

Most importantly, this build is extremely fun and you more or less just make the enemy team disappear.

Mythic: I go shield bow every game because the other critical mythics don't really work with Rengar as you still play him like an assassin not an ADC with DPS.

Main Legendries:
Collector is an extremely good item for this build. It gives you LETHALITY, AD and an sick execute. This item is almost always my 2nd item after mythic.
IE is important for this build as it gives you free damage but it can only be built 3rd item or after because the passive requires 60% crit or more.
Navori is an interesting item because it gives good AD and cooldown and allows you to use your abilities more often. I find it a fun item to use but there are other options.
Mortal is there because of the crit and healing reduction but you can really put anything else there.
Boots: Boots are situational, attack speed boots are good if you don't need the armour or MR.

Other Items:
In the build section there are a couple more items that are listed and can be used in the main build if you think they work better for you or in your situation. The real interesting one is Essence Reaver. The new form of this item gives you sheen which starts doing an extra 400+ damage when proc'd. If you think you can weave Rengar's abilities well enough to use this item well then I think its a good buy.

I am a fan of hail of blades because of the burst potential or dark harvest for the same reason. However conq. does work well on Rengar if you are playing in lane so if you are used to that rune then go ahead.

I normally play this top lane because its more fun than jungle unless you are playing with mates. So top lane counters are really most tanks and some bruisers if they are good at 1v1s or have some form of a shield mechanic. Obviously the squishier the target the easier the matchup (except for vayne if she can quickly use condemn). For jungle its a similar story and champs like Zac aren't particularly fun at all.

If in jungle I would just play Rengar like you normally do. This build is rather expensive so you need to farm more than normal. If you are top lane I wouldn't be aggressive unless you think you can kill. This build gets better as the game goes on so you don't want to int early or it won't be a fun time.