Unless your a Bard, Paladin, or Sorcerer, Charisma is by far the weakest.
Constitution is the most underated in my opinion. It may not be flashy, but the extra HP can me life or death of your character. Plus it boosts your Fort save to boot, and is useful no matter class you choose.
Strength is probably overrated. Although it is essential for melee classes, it is often the dump stat of choice for casters.
Dexterity is a great ability. It boosts your AC, which is a plus for all classes. Even wearers of Full Plate should have at least a Dex of 12. Plus it also factors into Ranged attacks, Ref save (although Ref saves are only used for a handful, although fairly popular and therefore common, spells), and many key skills.
Intelligence gives you extra skills points. Not essential, but often overlooked as well. It is also a factor in many key skills. Plus wizards get extra oomph from this stat.
Wisdom is a decent stat, but only because it boosts your Will save. It is fairly week compared to the others, though. Unless your a Cleric or Druid, don't put too many points in this stat.