My personal thoughts.
1. Calvin Klein Collection. It's obviously one of the biggest brands in the world, very iconic. They've contributed quite a lot to careers of Christy Turlington, Kate Moss, Natalia Vodianova, Lara Stone and Suvi Koponen.
They've been doing exclusives for a long time now, and even if an exclusive rarely becomes successful afterwards, they are considered one of the most prestigious and most talked about. Maida & Rami are the CD's who also cast Jil Sander and Dior.
2. Alexander Wang. He's really hot right now so obviously everybody is watching his shows. He's having exclusives who are slightly more successful than CK's. He also loves to stunt in his casts and already had Gisele Bundchen, Natalia Vodianova, Karolina Kurkova and Shalom Harlow walk his shows. He's had big named models in his campaigns too shot by one of the biggest photographers.
Also, as head of Balenciaga he's quite influential as well. Anita Bitton casts his shows and she also casts Balenciaga and Louis Vuitton.
Alexander and Calvin are neck in neck though.
3. Marc Jacobs. He's one of the most iconic and powerful designers ever, who has launched quite a few careers. His shows in NY are usually a tastemaker for Louis Vuitton in Paris. Lately they feature too many newbies though. Katie Grand styles his shows and even if she's very influential as well, I hate the socialite/hipster models she brings into his shows. They do generate buzz. Michelle Lee casts his shows and she also casts Versace and Celine.
4. Proenza Schouler are slowly taking in the third place. They are currently one of the best designers working and their casts are quite exclusive as well. Quite a few veterans have returned to the catwalk for them (Liya, Sasha, Luca, Heather, Kirsty, Georgina). Ashley Brokaw is casting director and she's the biggest right now, also casting Prada and Miu Miu.
5. Michael Kors. He's the most commercial out of the top 5, and the biggest luxury brand of America. And he's the most consistent, with many top models walking for him. He's so loyal though, that many don't (read: nobody) stand a chance at scoring his campaigns. Once you do, you're provided with years of work. John Pfeiffer casts his show and he also casts Donna Karan and Victoria's Secret.
This is just my point of view.