I voted none are useless... whether it's the most, the least or whatever. You just can't stick a label like that on any of the core classes because what may be a very ineffective class in one guy's game may be the most optimal and superior class in someone else's.
Case in point: A bard may be ineffective in a hack n' slash game dominated by barbarian and fighter classes Power Attacking and Sundering everything. His spells here and there and Inspire abilities will help the party, but he'll never really shine on his own. He's a supporting character. Same bard in a campaign heavy on role-playing, diplomatic missions, and scenarios involving finesse and tact make him the most optimum class to play.