Hmmm, no listing of the other 4, so I will assume Fighter, Cleric, Rogue, and Wizard as the standard mix. I will also assume no duplication of classes.
Of the choices given I consider Sorcerer to be a substitute for the Wizard not an adjunct to, so I will leave him out of consideration.
I would go with Bard as my first choice, he's a decent all rounder, and his abilities help the other PCs rather than being all that powerful in their own right.
Ranger is not listed at all, though he woud be agood all rounder to help in a mix of situations, he would be my second choice.
Barbarians fill a similar niche to the Fighter, though in a party where the Fighter has been replaced with a Ranger the Barbarian will add some close combat muscle while the ranger uses his bow.
Paladins fall a little too close to Fighter, I would consider them to be almost as redundant as a sorcerer in a party with a Wizard. His second rank clerical abilities also help with this, but he adds little that other classes can't do better.
Druid, an odd class, not quite able to replace the Cleric for healing and aiding others, but better in some areas, I would consider him for a fifth character, especially if the Fighter has been replaced with a Ranger.
Monks, I just don't have much of a feel for these folks, and have never had them as either PCs or NPCs, so I will leave them out of it.
The Auld Grump