I mainly play Kha'Zix as a mid laner, because I feel mid lane fits his role (and my play style) more, which is, an assassin who can push and roam during the laning phase. In teamfights I will jump in to assassinate high value enemies (ie. enemy ADC), bursting them fast enough so I am not in too much danger for long.
For this reason, my evolutions are:
Level 6 - W (Also max W first)
Level 11 - E (Max E last, only need 1 point early)
Level 16 - Q (Sometimes R, explained below)
The reason for evolving W for is for it's wave clearing ability. If I am behind in lane or getting jungle camped I can wave clear from a large distance quite easily without putting myself in danger. Evolved W makes you shoot 3 instead of 1 spike, and it applies your passive so at level 6, with a little bit of extra AD, it 1 shots all the range minions. You can also try hit the enemy champion with it at the same time for extra harrass.
On top of the easy farming, it allows you to push the lane fast. The reason for wanting to push fast is so your enemy has to defend tower, while you now have the option of roaming to your side lanes. Applying pressure to side lanes while your opponent is sitting there killing creeps under his/her tower is quite valuable (works very well with Boots of Mobility).
The reason for evolving E second (instead of Q second) is because Q evolution scales off the enemies health, and at level 11, most enemies won't have a big enough HP pool that the added damage is worth it. Instead, evolving E let's you jump further, so you can get better ganks off.
On top of the improved range of the jump, it will now reset whenever you get a kill or assist. Mid game and during teamfights this is very important, since you can get a kill and then straight away escape. The reset of the jump is very important in teamfights, since you'll be building very damage heavy with little defence, you'll need to escape as soon as you burst someone. Because you cannot guarantee that you will be level 16 before teamfights occur, this is best picked up at level 11.
Your last choice for evolution is either your R or your Q. I usually choose to evolve Q just because it does more burst damage and helps with tanky characters late game. If you feel you need to go a tankier build or cannot 1 shot burst their ADC, perhaps evolving R is better, helping you move around in a teamfight more. I personally do not evolve R in most games, but I can see that it could be useful in some situations.