But it's relative, the Monk and the barbarian can be pretty good actually with decent gear, And the witch doctor does pretty well except there are only 2 viable attack skills for inferno (Zombie Bears and Poison Dart).
The DH is OP and the WIZ can get OP with good items.
In inferno it's normal to play along in a party with 3 DHs or 3 WIZs or 2 DH and a WIZ, etc... That speaks books.
There are about as many Barbarians as Witch Doctors, Monks are rare.
None really suck.... and you can be 100% positive that Blizzard is going to balance the classes anyways, so trying to pick one just based on power /today/ is a totally pointless endeavor at best, and counterproductive at worst.
For what it's worth, my HC DH, Wizard, and Barb have all done quite well for themselves into hell (none are in inferno yet, but that's being balanced and toned slightly too, so guessing how you'll work in it is another pointless endeavor).