Personaly i prefert my Kha'Zix top because he can arrase the enemy very easily with his q because it have high damage and cost low mana but he can farm very fast too. If you arrase well you can get the enemy hp very low and still have alot of mana for jump with your E and take the kill by using your passive for slow Q on him and let him go with the ignite that finish him.

My personal preference for my evolution is to start at lvl 6 with the Q evolution so your are getting stronger in 1v1 and your passive have more range so you can arrase the melee enemy with your passive without using mana and he cant get close enough to use his auto attack for deal damage to you. For the second evolution at lvl 11 i am always going for the wing because it significantly boost your escape, engage and chase spell because of the very high range you can past through wall that a flash cant do, for the engage it good because it cause you to be less previsible because people wont have the time to get back when you apear that you already slow them and Q them and finaly for the chase it very good because for example an enemy get out of a fight the cooldown refresh refresh your cooldown after kill another champion so you can jump and slow the champion that try to escape and kill him fast 1v1 for come back in the team fight by rejumping in.

I choose those item because people oftenly focus Kha'Zix at the start of the fight knowing that his jump is in cooldown because he use it to go in and for reduce the number of time he cast his Q so i take alot of hp good defense and magic resist and the atma compense for the attack damage that you sacrifice for the hp. The good thing with this build is that you can survive easily but at the cost of your damage.

Pros: Kha'Zix is a very good ganker and engager and became even better when he evolve his wing because he can jump from very far and slow the enemie using is passive and doing vry strong damage to the enemie with his Q. Kha'Zix is a very good 1v1 because his Q make more damage when the enemy is alone if the claws are evolve it make 12% additional damage of the enemy hp and you can spam it approximatively each 3 seconds, but Kha'zix can do very good job in team fight too because he can go close the the mage or the range carry ad and kill them fast. Jumping on a carry ad or a mage and kill him before the team fight realy start can make Kha'Zix team win a team fight even if they are less feed.

Cons: Kha'Zix ult need to be combine with other spell if it want to be useful for chase or evade because his ult only still him for 1 second each 2 second so he can very evade on by spamming his ult and running he need to active jump active or for the chase Kha'Zix need to active his ult use a auto attack do his Q and reactive another auto attack and do Q again for always slow the enemy and deal high damage with the Q.