Originally Posted by PocketsTheGreat
... His Q deals extra damage based on how much HP they're MISSING. Q is by far his best skill, and the only one you should be evolving at 6.
I find that to be highly erroneous, but I respect your right to have an opinion. E allows for picking up quick kills that could otherwise escape you, and also allows for a duck-and-weave type of playstyle if you're so inclined. If you max your Q first, it would be best to evolve your wings because you're already hitting like a truck and you want to take advantage of any isolation bonus you can, and most of the time that's when they're running away or otherwise a good distance away from both teams. If you level W first, you want to evolve it to increase its effectiveness, as it becomes your main source of damage. If you're going a tanky deeps route, you'll probably want to max your W first and evolve your R at level 6 so you can get as many passive procs as possible; you're not going to be doing much damage with Q without damage items, but you have built in damage and slow via your ult + passive, plus survivability and speed boost for sticking.
You really only evolve Q at the later stages of the game when the percent missing health actually matters. When everyone's running around with 1000 health, if they're missing 400, that's only 48 extra damage when you're already hitting for ~340 or so, as opposed to the extra 55+ you could apply to your W by evolving it, instead.
But everyone has their own playstyles. Part of Kha'Zix intricacies.