WHO IS BETTER , WHO SHOULD I BUY

Those four aren't even comparable. There is an Assassin, an ADC, a fighter, and a mage. Which you should buy really depends on what role you want to play.

depends on your playstyle....

but

SHURIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIMAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Diana for ap burst, Kha zix for ad burst, quinn for adc/top lane, azir or mid/ap poke.

Really not comparable, just different types of champions.

Azir is fun in the mid lane

Diana is lotsa fun to play:O

I go for azir because he is cool and such. I want him too that's the second reason. And he would take a lot of skills but when masterd he is strong. At least that's what I heard. ;)