I'm sorry but I wouldn't start with YUI at all lol She's so boring
I would rather start with Utada in that case.
I would obviously introduce kuu, ayu and namie as well.
So Utada's Passion or the entire HEART STATION album, ayu's (miss)understood album, kuu's upbeat and rockish songs, and namie's stuff from her last album since some of the songs were in English.