For a party with 3 mages or 3 or 4 melees bard is the most powerful. For a soloist, it's probably the least powerful.
For a melee rogue I'd say you always want assassin. For an archer, probably not.
If you already have a bard and want a 2nd rogue, I'd say the 2nd one should always have duelist and then either assassin or ranger depending on whether he's a melee or an archer.
So basically the answer depends on what the rogue is going to be doing.