Gaultier Paris has a reputation among the young set for creating perfection for evening wear and a lot of highly laudable daywear. But I believe that among the ladies-who-lunch set, Lacroix is the favorite for evening wear and Chanel is a must for day.
Dior Couture is popular among fashion set like Anna Wintour and also many debutantes turn to Dior because, at the end of the day, no one's quite as fun as Galliano.
Ungaro, Valentino, and, to a lesser degree, Saab enjoy patronage from the Hollywood crowd during awards season.
I hope this helps.