CEO, Blackstone

His firm has nearly $100 billion in assets under management - private equity, hedge funds, real estate. And thanks to a spate of mega-buyouts, including February's $39 billion purchase of EOP and October's $26 billion acquisition of Hilton Hotels, Schwarzman - the "New King of Wall Street," as we called him in February - embodies the private equity boom. He could also embody its slowdown: The firm missed its earnings in the third quarter, and its shares have been falling for months. --Katie Benner