Yvette Nicole Brown was one of my favourites. Extremely focused and clearly took the responsibility of helping the civilian win the money with the appropriate degree of seriousness.
Martin Short is the undisputed worst, but Rosie O'Donnell and Mel B. were pretty terrible too -- the former wouldn't stop shrieking AND got shit wrong, and the latter knew nothing AND dressed like a goddamn moron in a strapless bustier jumpsuit and spike heels.