Wow... I don't know what all you guys are ranking your sportsbooks by, but everyone is leaving out the largest operation in the world - Sportsbook.com/SportingBet, Plc.
If you compare them with BoS in 2001, they're over twice as large (by bet volume in dollars, bet count, or customer base). They're also a public company on the London Stock Exchange, have a terrific (and large) sub license base (Sportsfanatik, Wallstreet.com, etc), and one of the biggest thought leaders in the industry heading up their N.A. operations.
If you look what they've been doing as far as marketing and brand building, they're in a far better position than anyone if online gambling should ever become regulated - and it's who's left standing AFTER regulation that will determine what the future holds.