Absolutely Zero Reason to pick Rengar over Kha Zix

Rengar's one shot damage would need to be A LOT (a lot lot) higher than Kha Zix to be a non-troll pick.

Kha Zix is just better in absolutely every single way possible in practice.

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Pre-6... Kha Zix has mediocre to strong Ganks/Counter Ganks in all lanes. His mobility is completely unrestricted and he can jump strike from anywhere.

Pre-6... Rengar has stronger burst combo ON PAPER. However, his enemies have to be stupid enough to go near bushes for him to do anything. He has to gamble on the enemy not understanding Rengar's kit. His Countergank/Gank is sub-par because his mobility is entirely restricted to bushes.

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Pre-6... Assuming enemy overextends and Kha Zix walks into lane from behind. Enemy Laner: "OMG I AM FUCKED. He's gonna auto-Q-auto me, immediate Electrocute... and if I flash away he'll jump after me and repeat until I'm dead or super low."

Pre-6... Ennemy overextends and Rengar walks into lane from behind. Enemy Laner: "What the fuck are you gonna do bro? Throw a Bola at me? I'm going to walk away from you and away from bushes so you can't jump after me."

This lack of mobility on Rengar translates into another important weakness. If you're looking to clean up and pick up kills (snipe off wounded enemies) in the early game... Kha has a higher probability of picking these up. Rengar's gotta walk, he's basically as effective as a Master Yi when it comes to early pickings.

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Kha Zix pre and post 6 can easily invade the enemy jungle with near impunity. His on-demand jump and invisibility means he can get away with sustained guerrilla warfare/camp stealing/assassination in the enemy jungle.

Rengar? Nope, not really. Its too risky. If he's cornered, he has ZERO escape. Dead cat. You dun goofed. Unless you're a Rengar one trick, ward over some wall to see a camp and then jump from some strange bush at the edge of the range to escape 2 enemies but come on...

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Post 6.

Again, on paper Rengar has a ton of damage but a lot of it is gated behind his Bone Necklace mechanic. Since he has a hard time getting kills pre-6 (against competent players who understand his kit), its a very strange design.

Now let's say we assume he somehow picked up 1-2 trophies. Now he can just Invis and then 1 shot ennemies with 1 or 2 necklaces right? Again, it sounds great on paper but in practice you have no way of escaping after you've 1 shotted the dude. There is no outplay/escape potential (its very limited even if it exists). In practice, with the same play style a Rengar is going to die A LOT MORE than a Kha because he's just got no escapes (either that the Rengar will just be too safe and not go in). This means that a lot of kills Rengar picks up (if he does) will mean less for his team because he's giving away gold and exp too every time he dies.

Meanwhile, Kha has way too much outplay, mobility, and escape capability. In the hands of a skilled player, he does SO MUCH MORE than Rengar. And while his 1 shot is less powerful, in practice, because he's more likely to pick off kills pre-6 (so he's in a stronger place at this point in the game) his 1 shot is often close to equal to the Rengar who is gated by his Necklace and a super clunky early.

The biggest deal though is that a skilled Kha is going to have a very low death rate while having a great KDA. The enemy's not getting tons of gold/exp from killing him. Meanwhile, any decent team will kill Rengar when he goes in (best case scenario is the Rengar kills 2 people before himself dying).

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TLDR

Rengar needs A LOT MORE burst if he's going to be a viable pick because Kha is just flat out better.

His pre-6 is awful, he's got no escape when he does go in. If you're going to power farm to 6 and then delete 1-2 enemies before dying every time your R is up again there is no reason to pick Rengar. Master Yi is WAY BETTER at doing that.