Out of these three Very Irresistible would be my choice. I think Coco Mml. may be with too much patchouli based on her other favourites. I would go more for fruity and floral scents.
But there are so many other lovely fragrances out there that she may like as well -
Euphoria (Calvin Klein)
Pure Poison (Dior)
Britney Spears' Curious and Fantasy
J Lo Live and Miami Glow
Burberry Brit Gold
AnaisAnais (Cacharel) - that's a classic, but may be too floral and not sweet enough for today's 18 year olds. But it's a very pretty, clean floral with a hint of fruity and green notes and plenty of lillies...