She is wearing Valentino Pre Fall 2016. (source)
Click here to view last year cover featuring Jessica Chastain, here to view 2014 cover featuring Jennifer Lopez, here to view 2013 cover featuring Cheryl Cole, here to view 2012 cover featuring Cameron Diaz, here for 2011 cover with Daisy Lowe, and here for 2010 cover with Gwen Stefani.