1. Con
2. Wis
3. Dex
4. Cha
5. Str (with standard encumbrance rule)
6. Int
I wouldn't rank Con that high because it doesn't do anything in the majority of circumstances. If you're level 10 and never drop below 11 hp, then having a 14 Con instead of a 12 Con was irrelevant. Additionally, Con saves aren't that common, aren't as deadly as they were, and unless you hit the DC on the nose having a 14 instead of a 12 is irrelevant so it only matters in 5% of saves. At low level I'd also rank Dex higher than Wis because it can completely replace Str, has several skills that are more useful, and Dex saves are much more common than Wis saves. At high level, Wis saves get much more important and so does Perception. I'd rank Cha higher simply because it's
the spellcasting attribute.
I'd rank them:
1. Dex/Wis (tie)
3. Con/Cha (tie)
5. Str
6. Int
Dex has, pretty consistently, been
too good throughout all of D&D history. Str was only too good as long as percentile strength was included. Since that has been dropped, it's basically been mediocre in and of itself at the levels PCs are capable of.
IMO, the reason Con exists separately from Str is so the game can make small or tiny monsters that aren't automatically frail, and can also make durable monsters that don't automatically hit like a truck. In other words: the problem is that HP is tied to an attribute, and you need those separate on some level to build interesting monsters.
I think the only reason Wis exists separately from Int is so the game has an excuse for Wizards to use a different attribute than Clerics. And I think Charisma remains a stat that's almost entirely superfluous. Indeed, I think it was put in the game in part for players to use as a dump stat. In whatever attempt game designers have made to make Cha not be useless, they've continually expanded what it means while simultaneously restricting Wis and especially Int. So now they've put lots of things into it that just really don't belong there. Persuasion could be Wis or Int. Deception could be Int (it's out-smarting someone, after all). Even Performance could be Dex or Int depending on it's nature. It's not a good design for Bard, Sorcerer, and Warlock (and Paladin) to
all use Cha. Beyond historic reasons, there's no reason to have six attributes over, say, Str, Dex, Con, and Mind, or Str, Dex, Con, Int, and Wis. Even then physical stats are all easily understood. The three mental stats are a cause of confusion and argument. How is will (Wis) different than force of personality (Cha)? You
can make that distinction, but it's contrived. There are many nonsense assignments of Int/Wis/Cha. You can describe a character with those attributes, but often it's
narrow with only one possible character personality. Attributes shouldn't be doing that. Charisma is particularly guilty of this, because it's so closely tied to personality.
5e's six save system is a clunky way to introduce Strength saves into the game (let's be honest, that's all that was really missing from Fort, Ref, and Will). I'd rather see five saves: Brawn (Str), Reflex (Dex), Fortitude (Con), Will (Int/Wis/Cha). Or maybe Will (Wis) and Reason (Int). But I don't see any reason for Cha to be a save. The unbalanced part here is that there are
three stats in the game to represent
two characteristics: knowledge/reasoning and social awareness.