For me it's
Lulu > Vex > Poppy > Tristana
It's not that I dislike Tristana, she's just like the bog standard yordle girl. She's energetic, but not as fun and whimsical as Lulu. She's friendly and nice, but Poppy fills that niche of a girl you'd want to take out on a date better. And Vex is Vex, so sex.