Hi guys!
I'm Tuphor, a silver 1 on EUW who likes to play The Void Reaver.
This being my first guide, I would love it if you would comment after rating and if you have tips for me I would love to hear them!
Kha is a strong mobile melee assassin who excels at jumping in, deleting a squishy and jumping out again. Moving on to the pros and cons

Pros

Very fun to play
A tribute to the Alien movies
A terror in Solo Queue if played correctly
Can 100-0 isolated squishy in literally a second

Cons

Squishy
Can fall behind pretty hard
Gets countered by stealth reveal
People tend to rage at you for playing an "op champion"

Here I'll explain my choice of runes.
NOTE: MY RUNES MAY NOT BE OPTIMAL. I ONLY SPEAK FROM MY OWN EXPERIENCE
I think the Armor Seals speak for themselves. Less damage from AD champs, less damage from minion, less damage from turrets
The flat magic resist runes are there because I might play against an AP champ and I'm too poor for multiple rune pages.
The armor pen are there for more damage against champs, and to help with lasthitting there are some flat damage quints

The masteries are pretty standard for a melee assassin.
Double-Edged Sword for more damage output, CDR to use abilities more.
The masteries that give you more AD are pretty standard as well, just like the Executioner, because you like low-health targets. Spell Weaving and Blade Weaving because you rely on both AA's and spells. The armor pen/magic pen mastery is a must on every mastery page with 21 points in offense. And if you can get 3% bonus damage with 1 point, why not?
Defensive masteries, health regen because +5% bonus resistances is stupid on Kha. We're not building resistances silly. Block and and Unyielding for dueling, and the bonus healths for surviving longer.

Flash is pretty much standard on toplaners, it's your "get out of jail free" card.
Ignite is the standard for assassins. If you burst is not enough and your enemy gets away with a sliver ignite is your hero. You could take teleport for splitting and roaming, but Kha can do that pretty good without teleport. Besides, splitpushing is nice, but it's not your main goal, and before you have evolved your jump or bought your mobility boots you shouldn't be splitting anyway. As for roaming, yeah, teleport is nice, but I will always take ignite over it.

Your passive is Unseen Threat. It gives you a buff EVERY TIME YOUR OPPONENT LOSES SIGHT OF YOU.
This means you can discover wards by checking if the buff refreshes. The buff gives you bonus magic damage and a slow on your next autoattack. It's a pretty neat passive for trading and it gives you better chase potential. It is also the reason why you need to use your ult WHILE FIGHTING.

Your Q (or A or Z whatever you prefer) is Taste Their Fear. It is the sole reason why people need a new diaper when spotting you on their own in the jungle and your bread-and-butter damage ability. It is a single-target nuke in basically your AA range. If people are isolated, it deals increased damage. If you evolve this, it deals bonus damage based on the targets % missing health, and both your AA and Q range get bigger.

Your W is Void Spike. If fires a spike in a straight line. It explodes in a small area to damage and slow enemies in the area. If you are in said area, you are healed. This gives you some nice sustain and chase potential. If you evolve this it fires 3 spikes instead and the slow is increased.

Your E is Leap. It's an AoE jump, and your gapcloser. If you evolve this your range of Leap increases drastically and if you get a kill or assist it resets. This gives you your "jump in jump out" potential. Evolving E also turns this skill into an awesome DUNK.

Your Ult is Void Assault. It speeds you up and gives you short invisibility. This also resets your passive. You can activate this ability one more time after the first during a short period. That means you can activate it 2 times every time it's off cooldown. If you evolve this you get another cast. That means you can activate it 3 times. Also it gives you 50% damage reduction wihle invisible.

I take Q first for level 1 shenanigans. If you can kill the enemy laner because he is on his own it's really nice.
I take W second for lane sustain.
E comes third so you can escape from ganks or go in hard.
I max Q first because like I said it's your main damage source. I also evolve it first so IF the enemy gets low they're DEAD. Nice ult Mundo and BYE.
I evolve E second, because it gives you ridiculous jump range and resets
I also max it second because damage is more important than sustain for me. And makes your dunk land harder, which people don't expect.
I max W last, because the sustain isn't THAT important and it also got weaker.
I max R whenever I can, because it's my ult, derp.

This is a debate that started when Riot nerfed the W evolve and buffed the ult evolve. Before that evolving ult got you 9 reports. But not now. The answer to the question is very simple: look at your enemy team. If they have big AoE, you evolve your ult, because they can still kill you without seeing you. If they lack AoE and instead rely on lockups and targetted abilities/AA's you take W becuase they can't really hit you if you're invisible anyway.

So for starting items I usually take boots and pots.
Why?
Well, boots give mobility, which kha likes, and the 4 pots is also very nice.
Of course you can start doran's shield if you are a ***** and think you need the defensive stats.
But starting items are not that important, you just need to lasthit well and stay alive.
What IS important if your core. Take Tear first, the sooner you can start charging it up the sooner you get the muramana for *****slaps all around. Brutalizer speaks for itself, and Mobility Boots give you more roaming potential. After that, get LW for more penetration and Cleaver to shred through even tanks. Trinity Force is brutal against towers and every living being on the rift, so get that too, and BT for some sustain. Get Muramana AS SOON AS YOU HAVE 750 STACKS. Not sooner, but definetely not later.

So a lot of people get the Hydra on Kha and will flame you if you get BT. But you need to ask yourself, what is Kha best against? Isolated targets. Kha is best against Isolated targets. What is Hydra best against? Grouped up people. The best thing is: the AoE of Hydra is EXACTLY as big as the range of the isolation of Kha's Q. THE ENTIRE ESSENCE OF HYDRA IS OPPOSITE TO THE ESSENCE OF KHA'ZIX. Don't be a sheep, think for yourself.

Early Game
In early game you need to farm up and avoid dying mostly. If you can, bully your enemy by hitting him with Q while staying in between minions. ONLY JUMP ON YOUR ENEMY IF HE IS ONE 30% HIS HEALTH OR LESS. If you jump him before that, you blow your one escape on a kill that you might end up not getting.

Mid Game
This is where Kha'Zix excels.
If you executed him well, you farmed up, you might even have gotten a few kills! Either way, this is where the skirmishes start happening. Try to walk with your jungler, so you can jump unsuspecting enemies in small fights. If dragon is up, do it with your team, dragon is always isolated so BE THERE. Your Q is vital in such situations. You need to constantly catch people out, so ward their jungle and pay attention to side lanes. Nothing is funnier than blowing up that splitpushing Doublelif- I mean ADC.

Late Game
What you want to be doing in your team is mostly waiting for an engage, then jumping on one of the enemies carries. Remember, you thrive in chaos. Try to get someone on your team as Shyvana who jumps in the enemy team and gets them to split up. Whatever you do, don't jump into a tightly grouped enemy team. You need to pay attention to Baron, same story as with Dragon, your Q is vital. Also, you can start shredding turrets with Trinity Phreak now, so you can start splitpushing hard. Another reason why you can splitpush pretty well, is that your ult and evolved E give you a lot of escape potential.

This was my guide to Kha, please rate, comment and maybe give me some times.
But over all, good luck with this amazing champion and shred some Teemo's for me!