Taylor Momsen > Blake Lively > Leighton Meester > whoever plays Vanessa as far as beauty is concerned.
However, Blair represents exactly my personal idea of the perfect kind of girlfriend: a rich, classy, snobbish, smart, witty, relentless, manipulative, plotting, hot and sexy bitch. Too bad it's impossible to find girls like that. Episode "Victor, Victrola", where Blair gets out of control, is one of my favourite.