In my perspective kha'zix is op. For all you disagreers out there, too bad. Let's get going, no useless bs here.

Don't say this is a general ad carry build, because it still need the skill to win.

To start off, buy boot of speed and a doran's blade. Get enough gold for a second doran's and berserker's(better to have enough gold for both, but its ok if you have only enough for a doran's). Then get a zeal for attack speed. Do not build Phatom Dancer just yet, you need to increase your attack damage in order to get more kills. Build up a Blood Thirster for more lane sustainability. After the blood thirster is built, finish up Phantom dancer for catching up with AD/AS ratio. Build an Infinity Edge for critical damage and percentage. Build up another phantom dancer for 80%+ crit. probability. Finally buy a Frozen Mallet for chasing down an enemy and health.

Priority order: R, Q, W, E.
When build Kha'Zix AD, Q is his butter for his bread, use your Q to attack, and finish off, or even ks others. W, is his second damage source, slowing down enemy and farming minions. E, is useless on attack, but incredible on chasing down an enemy or escaping. R makes Khazix invisible twice (three times if evolved) for a short period of time, use this time to escape using E or flash, or, dodge aiming of enemy champions. Evole Q W and R for more OP-ness.

Combo: Q ->W ->Q ->E ->Q ->R ->Q ->W ->R ->Q ->-(if they aren't dead) ->W ->R ->Q ->E ->Q

Gank with team and get a first blood for Gold and a good start, last hit(of course) the minions, ignore the Alters, let your team mate get them for you. Remember that <<<<TURRENTS>>> are <<<MORE IMPORTANT>>> than <<KILLS>> than <BUFFS>. The point of the game is to destroy not kill like a psycho(it IS fun to do so, lol). Use Speed buff for faster chasing and running. Thats is all there is to this guide, good luck pwning the other team!