Hy, my name is Ady -ign : D3athKnight
I've been playing LoL for about 4 years starting from Bronze 3,now i am Platinum 5.
Kha'zix was one of my favorite champions since he appeared, I can say it was love at first sight.

Pros

+ High Burst with Q Damage
+ High Mobility + Resets
+ Easy to Farm With
+ Very Low Cooldowns
+ Incredible Duelist

Cons

- Very Fragile
- He Spikes at level 11
- Mana -Unlike Rengar: His rival.
- Super Low Ranged Damage
- Requires Good Positioning

Ignite and Flash are what I run on Kha'Zix in 100% of my games. In my eyes there really are no other choices. Ignite helps your dueling power scale well in addition to giving you the most damage possible early which help how allow you to snowball.

We want to go flat AD Marks/Quintessences, so we can win early trades. You can also go with flat armor penetration if that's what you prefer. I personally like the AD ones, because it helps me farm better.

Armor Seals should tone down the enemy's auto-attacks in case he is range.

Magic Resist Glyphs gives you that much needed magic resist.

Once you purchase Last Whisper, your damage will spike incredibly. Giving you 35% armor penetration along with 40 attack damage, it's a very low cost item for what it gives

This item is the build off from Tiamat. It gives you extra attack damage - 75 - and 15% life steal, furthermore the active now applies with life steal. And you can cast the active mid Leap!

The Brutalizer should always be your first main goal as Kha'Zix, It gives him 1Damage, 2ArmorPen, and 3CDR! Which are 3 core aspects that Kha'Zix needs, to be a threat

I only buy these when the enemy has an AP heavy team or Im getting dunked by an enemy AP mid laner/jungler.
It gives you good Magic Resist along with 35% Tenacity.

this item is great for teams that are filled with auto attack reliant champions such as Vayne, Tryndamere, and Master Yi.
it gives you a great amount of Health, Armor, and a great active that can help a lot in teamfights, as it slows the people around you.

I love this item for its amazing Health Regen, and combined with Ravenous Hydra you will never have to recall and last super long in fights (as long as you have blue buff lol).
Gives a great amount of Magic Resist and Health as well.