Without question, Constitution is the best stat.
For Barbarians, Constitution not only increases the capacity of the cast to fulfill its main purpose (damage sponge); it also increases the duration of raging.
For Bards, Clerics, Druids, Wizards and Sorcerors, Constitution increases the capacity to do combat casting; at low levels the size of the Con bonus to Concentration makes a considerable difference when it comes to the viability of casting in a melee situation. For a 5th level wizard, the difference between a Con of 10 and a Con of 16 is successfully combat casting (with the feat) 3rd level spells 70% of the time and 85% of the time.
For Rogues, Bards, Wizards and Sorcerors, high Constitution is absolutely crucial for basic survival. Remember that for D4 hit die classes, a Con of 16 will provide over 50% of the class's total hit points after first level. Similarly, for D6 hit die classes, a Con of 16 will provide 46% of all hit points gained after first level. For a 7th level Wizard, the difference between a Con of 10 and 16 is 19 hit points versus 40 hit points.
In addition to all this, Constitution is the main attribute for all Fortitude saves.