Not trying to start a flaming war here, but since the question was asked.
My vote is for three coaches:
Dick Umile and
Enrico Blasi, two peas in the same pod. They have great teams, yet are not able to pull the teams together in a cohesive manner when 'the big game' counts. The third worst coach (IMHO) is
Ted Donato and you can read the comments on the Harvard Hockey Future is so Bright thread. Again, Donato recruits top-notch talent, but is not able to coach them to win. For two years in a row, with Donato recruits nonetheless, he's been unable to pull out a double-digit win season.