Who is Number... 5?

[Guilt Puppy]

5 is Bard. Definitely.

It's not the jack-of-all-trades bit. You need a diplomat, and none of the "core four" are really built to be diplomats. The Cleric might be able to pull it off, but it's better to have someone to specialize in it. The Bard is optimized to fulfill that role, just like the other four are built to fulfill their roles.

Not to mention that any serious group of adventurers needs someone to record and tell their tales.

The jack-of-all-rades bit is nice too. Combined with the songs, the general effect of the Bard is to improve each of the four core roles a little bit. But that's a throw-in. The diplomatic part is what's crucial.

 

[Dr. Strangemonkey]

Druid or bard are great calls. Get that healing factor in and they are both information gathering machines, though they have very different methods.

Also, Dwarves are never bad to have in a party even if they aren't a class. They are indespensable in any sort of dungeon or castle.

 

[Left-handed Hummingbird]

A party consisting of a Human Cleric, Dwarven Fighter, Elven Wizard, Halfling Thief, and a Gnomish Bard looks pretty good to me.