These are the summoner spells that you would take on Kha'Zix. Flash is always a must because of the gap closing potential as well as the escape potential, and there really isn't a reason to have anything else. Smite is essential for jungling, whether just taking camps, using red smite to duel, or stealing Baron, smite is always a must.
First thing I want to do is clear a VERY common misconception of the trade off between Dark Harvest and Electrocute: In almost every situation, if you know what you are doing, Dark Harvest IS the better option. The empowered auto helps your dueling more, increases your relevance in mid/late game, and snowballs harder. The only reason to EVER take Electrocute over Dark Harvest is if you're in a position where you won't be fighting the enemy jungler, such is if you are against: , because you want to try to avoid these champions, as they can outduel you very easily.
PASSIVE-When Kha'Zix is not visible to the enemy team, he gains Unseen Threat, causing his next basic attack to deal bonus magic damage and slow. This has great synergy with Dark Harvest as when they combine, your first auto in your combo (after using Q) will be the killing blow.
Q-Taste Their Fear. This is an auto reset with slightly bigger range than your auto attacks that will be the first move of your combo. If you choose to evolve it, it will also gain 45% CDR if you use it on an isolated target.
W-Void Spikes. A skill shot that hits enemies for physical damage, and also heals Kha'Zix if he is in the explosion radius. If you choose to evolve it, it will shoot three directions, be bigger, and grant a bigger heal.
E-Leap. Kha'Zix's main gap closer and ganking tool. You want to use this after the enemy uses their flash/main escape. If you choose to evolve it, the range increases and the cooldown resets on kill or assist.
R-Void Assault. Activating this will grant you true stealth for 1.5 seconds, as well as reactivating it within the time it allows. If you evolve it, you also gain stealth when going in bushes out of combat.