Rogue.
They're one trick ponies because their main abilities come up infrequently.
I can count the number of traps I've faced in the last 10 sessions I've been in on 2 hands, and the ones that were significantly threatening on 1.
Sneak attack requires a lot of babysitting. Cast a mid level spell on them, help them flank, specially craft Strike items so that when they face one of the half million creatures immune to sneak attacks they aren't worthless and still realize that when you face a creature with no end runs around sneak attack immunity almost any other class would be far and away superior.
Its a bad thing to realize that when you are low level and hacking through undead, plant and elemental baddies that you wish the rogue was a WARRIOR!
The thing is, the bard is always contributing but the Rogue can get into a situation where it is almost impossible for him to contribute.