Problem with evolving his Q is that the damage bonus is only against isolated targets. If your enemy is even remotely smart, they won't be dueling you early on away from their team, minions, or jungle creeps, all of which prevent isolation. Evolving his Q is basically gambling. If you can keep finding people out or the enemy doesn't know how to fight Kha'zix, you can wreck them. If they prevent isolation, then the evolve is entirely wasted. It's very hard to take advantage of during the laning phase.

W drastically increases his farming power and poking potential to soften up the enemy before a team fight. Poke, reset passive, poke, etc.

I usually do W, E, then Q as my Q is evolved for late game squishy hunting when the map is warded and people are roaming between lanes to farm creeps in between objective pushes. You then jump them in the jungle and blow them up or to jump on runners. Until then, it's kinda hard to take advantage of.

You overestimate people, lol. But OT, if you are winning lane, it shouldn't be an issue because W pushes too hard when you want to freeze at top when winning to deny farm. With q you can poke pretty well when they go in for cs, goes melee range to get cs you q him. Mana isn't much of an issue with q as well. Only problem I see with evolving w and using it to farm is betting on a bad jungler that can't see an opportunity against an extending kha'zix. It's some good amount pressure from that. Evolved Q comes with extra range that helps you harass when they come in for cs. When chasing in 1v1, q's isolation is almost guarantee so if you win a trade and they run backwards and you have room to go before tower, you aren't wasting the evolve bonus most of the time.
Edit: When winning the only added benefit from w is the zoning mechanism from increase sideways range from w. However, that can be negated because you can use fear to zone them by jumping on them with any chance you got. If they stay far enough from creeps to keep you from harassing with q but enough to get exp; there is a good chance they are also isolated.

Usually W>E>Q but sometimes I go W>Q>E or W>E>R

When I play him I evolve q, w, e and rarely r

Reason being is my item choices

I 1st buy shoes and 3 pots
then upgrade shoes and buy a phage
then i get brutalizer
then i get trinity force
then either a bloodthirster or a hextech gunblade
then guardian force
then either a bloodthirster (if not gotten before) or infinity blade
then upgrade brutalizer to ghostblade

Reasoning for these choices you may ask? Especially why i get some AP?

Well, his passive deals bonus magic damage based off your AP
Also, the amount of health recieved and damage dealt from your W attack is based off your AP (remember void spikes uses up unseen threat)
Getting the Hextech gunblade has been very usefull for me, especially when you need an unexpected heal from your W in a fight, (Evolving W will give you crowd control slows and if I am correct it heals you more because 3 explosions = 3 chance to heal you, dont quote me on the its hard to tell)

In general I never upgrade my ult, more times than not W is waaaay better to evolve, Q and E are a must though!

With an upgraded ult more times than not youll get owned by team fight aoe anyways and your ult is not going to save you. Khaz's ult is not very good, you weill have rather upgraded your W and have some AP so it actually does damage (because it deals the slow and magic damage from unseen threat and if close enough heals you too!) I can get some champs (lulu comes to mind) to 1/2 health with 1 evolved W attack if they are a couple levels below me.

A good Khazix needs to have a good farm, Poor farm = dead Khazix

Anyways hope this helps, Once someone told me what I'm telling you, I have been kicking some sweet ass with Khazix, though Khazix is very hard to use in teamfights if your team has no good tank or CC

Yeah, but if you don't evolve it, you're depriving yourself of a 4th ability...


Also, just ignore the R. It's by far the least impactful evolution of the 4. E is a must, unfortunately. And Q is situational, it's kinda useless in teamfights unless you're specifically cleaning up or know your enemies are idiots who don't understand the concept of staying as a team.