Sorry. Got a bit excited at the prospect of time alone with a certain Mr. Black.
For all round general survival: Dumbledore
For intelligent conversation: Hermione and Remus (with a whole loada wolfsbane potion)
For protection: Harry and Hagrid
For a laugh: Fred and George
For a challenge: Severus
For populating the island: SIRIUS (sorry guys)

Ummmm what else?