Nacho like super said the evolutions of Kha'Zix depend on the game. I personaly play Kha'Zix all the time despite my Talon pic ( i like the way he looks ) I usually have to upgrade leap first because the people counter Kha'Zix by either ganking him or getting champions with gap closers. Also you need to go more in depth in a lot of areas. First items, you need to make a section for situational items. There are more than 4 situational items for Kha'Zix shuch as Maw of Malmotious and Ionian boots of ludicity. Also for your skill sequence, you don't use leap because...
1) You loose the only power that is a gap closer, so if they flash your screwed.
2) If the person is leading you to his teammates unless you get a kill your screwed.
3) I only recommend for you to use leap is if you can kill the person with a quick leap, taste their fear combo.
Also it is good to use your special while fighting because...
1) It activates unseen threat.
2) You get a moment to ***es your situation A.K.A fight or flight
3) Usually after using your special your taste their fear is off of cooldown.
4) (If you have the evolved form, which I always recommend) Any damage over time effects do 50% reduced damage (i think im doing this off the top of my head) and you can get the unseen threat power up to 3 times.
Kha'Zix is meant to kill fast so the teammates don't come and help in time. Usually in teamfights stay back and throw a couple of void spikes out their that's it. When you see that someone is at below 50%-30% health go and junp in and kill them, try to come in from the bak because usually this person will be running and not staying in the fight like a ****** unless he is the tank, NEVER ATTACK TANK UNLESS YOU ARE 100% SURE YOU CAN KILL HIM! Go for adc apc and squishy support players. If you know what the special of the enemy is ( which I recommend you start learning if you don't know all the damage and support players ultis ) and they've used it jump in because their main source of damage is out and if your playing right iy didn't come anywhere near you.
Take this into cinsideration plz, I'll try to help as much as I can.

Good Luck!

Merry Christmas to you too cya

I will be away for 3 week (going to Thailand :D) just so you know why I won't answere if you write something.
I wish you a Merry Christmas!

I play on both Nordic/East and West, my name is Superhoben (not so logic either).
I have a free day tomorrow (teachers are busy with setting grades) so maybe I will play a little then.

I play in EU West i think, my name is MagicMushrooms69 (dont questions my logic, and no i dont have teemo). I play Fiora, Kha'Zix, Orianna (AP/AD), Akali and AP Yi (dont remember any else). What about your?

Oh, and something that don't belongs here actually but on which server do you play and what's your name in game?
I would like to play with you.

Really nice! Now it deserves my upvote (just gave upvote). Great improvments that turned this guide really good. When you have time add a sections about specific lane match-ups.
And again really nice guide now

I didn't, i just made a total upgrade, runes, items, masteries (obviously), rengar section upgraded.

Why haven't you responded yet? Have you given up?
You will get this guide good, sure it got a couple of down votes which is because of the lack of tankiness and runes but that's very easy too fix. What's hard is to write good text which you already have done.
Just don't give up, you will make it!

There are so many issues i see here.

Let's start with runes. armor marks are used when mpen, apen, or ad will not work for the character (such as supports). Armor is much more efficient in seals, and that health is easily substituted by items. AS glyphs? 6% more AS? that is extremely inefficient. In fact that isn't even noticeable. MR or even CD would be more efficient here. Crit damage is one of the worst rune stats, especially for their costs. 13% crit dmg? o you crit for 400 while having 200 ad? now you crit for 426. 26 more dmg per crit is not that much, judging by how you can get more ad or apen which is a lot more consistent.

Build. Sword of the Occult with no way to survive? those stacks will disappear faster than they appear, except in extremely low level play where players tend to run after being hurt instead of realizing they hurt much more. That being said, Kha'Zix is melee. By the time you get into melee with no tankiness, you're already at least half dead. The stealth will only protect you for a very small amount of time. Literally if you walk into a teamfight, you will be blown up before you can reach a squishy target, since a team will have some kind of cc and the squishy target can either outright burst you, or kite you all day.

Lategame it will be close to impossible to keep a fully stacked "sword of the occult", especially without any tanky items. Sell it for Guardian Angel, Frozen Mallet or Warmogs Armor.
Without any tanky item you get stunned/exhausted and die before it ends and an ad carry like Ez and Graves will be able to solo you if being same fed late game (Ez use E to get away a bit and kill you before you reach him, Graves just kill you straight off).
Stealth from ult won't be enough, especially not after that their tanky bought oracles.
Switch Marks and Quints:
Each armor Quint provide 4,26 and as you have 3 it becomes a total of 12,78 armor.
Each armor Marks provide 0,91 each and as you have 9 it becomes a total of 8,19 armor.
At the same time each Crit damage Quint provide 4,46% crit damage, becoming a total of 13,38% crit damage.
While each marks provide 2.23%, becoming a total of 20,07%
Current bonuses from marks and quints:
8,19 armor and 13,38% crit damage.
If you switch them:
12,78 armor (4,69 more) and 20,07% crit damage (6,69 more)
I know that it's only details but still, those details can be the diffrens between dying or getting a
kill.

The Q cooldown os 3.5 secs and thank you ill do that.