Well, from a crunchie standpoint, I'll go with Cleric, Rogue, Fighter. If you include Psionics, replace Fighter with Psychic Warrior.
Reasons:
Clerics allow groups to handle encounters that are too difficult to handle without a cleric. They also include many save or be neutralized spells, plus they can be adequate combatants. No other class is more capable of dealing with undead. The selection of domains allows a cleric to fit a specific role in the party not already covered by another PC.
Rogues allow groups to bypass hazards that would be difficult to overcome without a rogue. Their sheer number of useful skills along with the large number of skill points to allocate to them means there are few situations where a rogue is not useful. They are also capable in combat, one of the few classes that can keep up with the melee damage output of Fighters, though only against opponents that are vulnerable to sneak attacks.
Fighters, few groups can last without some kind of main combatant. While other classes can fit this role, none do it better than the Fighter. With the number of feats a fighter gets, he can be useful in a diverse number of combat situations. It is difficult for other classes to keep up with a fighter in terms of damage per turn consistently throughout the fighters career largely due to Weapon Specialization. Fighters also have a decent life expectancy; with better hit points than most other classes, they are less subject to dying from incidental damage.
Note that all these classes also offer a great deal of versatility when building them, allowing players to tailor the class to suit the campaign even without resorting to multiclassing. This is a powerful "hidden feature" that many other classes **coughmonkcough** don't have.
sotmh
