Of course, it partly depends on what your definition of "the worst" is. One of the threads on OTW a while back asked for the worst movie of all time. My answer was the first of the three Star Wars prequels (Episode I). Not because it had the worst acting of all time, or the worst plot of all time, or had the worst box office performance of all time -- but because it had so much to work with (money, actors, marketing, special effects, creative license) and ended up being just so colossally disappointing overall. It may have been a success at the box office, but it sucked horribly. Not as bad as some really terrible flicks, but it was the massive difference between potential and realization that made it the worst.
So there are some truly horrible coaches that didn't have a lot to work with and accomplished nothing (Hank Bullough), and then there are those who had considerable talent to work with and severely underperformed (Chan). Who was worse? I'm tempted to say Chan...although Hank was truly a trainwreck of a coach.