I voted "Druid," because, like several others have mentioned, the correct answer is "another cleric." In general, I like parties built this way: Healer, Warrior, Arcanist, Skill-user, Healer, Warrior, Warrior, Arcanist, Skill-user, H, W, A, S, H, W, A, H, W, A, H, W, A, etc. So a six-person party should be something like Cleric, Fighter, Wizard, Rogue, Cleric/Fighter, Druid/Fighter... or maybe Druid, Barbarian, Bard, Ranger, Fighter/Cleric, Mystic Theurge.
I believe Skill-users (Rogues & Rangers, maybe Bards) aren't as important to have in numbers, because any class can max out one or two of their skills to pick up the slack in a Skill-user-light party, and be just as good as a Skill-user class. A medium-BAB class can't replace a warrior as easily, and no class can replace a spellcaster without dumping a lot of resources into magic items. Of course, the core of the party I'm DM'ing right now is three rogues.