I like to analyze this in terms of what +2 to an ability buys in terms of equivalents feats to get the same advantage. I will avoid discussion of abilities giving a bonus to skills until later.
+2 STR: gives +1 to hit in melee (better than Weapon Focus) and +1 damage (+1.5 damage with a two-handed weapon) (at least half as good as Weapon Specialization, probably better than that), and improved carrying capacity.
So +2 STR is worth about 2 feats.
+2 DEX: gives +1 to hit with a missile weapon (better than Weapon Focus), +1 initiative (worth 0.25 feats vs. Improved Initiative), +1 Reflex save (worth 0.50 feats vs. Lightning Reflexes), and +1 AC (better than Dodge).
So +2 DEX is worth about 2.75 feats - as the last two abilities are defensive, it should probably be slightly less, e.g. see Sean Reynolds' feat point system:
http://www.seankreynolds.com/rpgfiles/misc/featpointsystem.html
So with that approach I would rate it at 2.4 feats. It's also worth noting that DEX doesn't necessarily improve AC that much, as heavier armors can compensate: e.g. Full Plate with DEX 12 gives +9 AC, whereas Chain Shirt with DEX 18 gives +8 AC (and max DEX bonus with a Chain Shirt is +4). So that brings DEX back more to the range of 1.4 to 1.75 feats for those who wear armor.
+2 CON: gives +1 HP/level (equivalent to Improved Toughness) and +1 Fortitude save (worth 0.5 feats vs. Great Fortitude).
So I would rate +2 CON as +1.5 feats. More than that if you don't allow the Improved Toughness feat and are in a high level campaign as it gives more HP (e.g. +20 HP is 6.7 picks of Toughness).
+2 INT: gives (level+3) skill points (this is worth (level+3)/5 feats in comparison to the Open Minded feat). For an INT-based spellcaster, it also gives bonus spells and +1 spell save DC (better than Spell Focus).
So I would rate +2 INT as between 0.8 feats (at 1st level for a non-INT caster), 2.8 feats (at 1st level for an INT caster), 4.6 feats (at 20th level for a non-INT caster), and about 6.6 feats for an INT-based caster at 20th. [Obviously, you should go for INT for skill points at 1st level, rather than later, as it gives more benefit if you have it from the start.]
+2 WIS: gives +1 Will save (worth 0.5 feats vs. Iron Will). For a WIS-based spellcaster, it also gives bonus spells and +1 spell save DC (better than Spell Focus).
So I would rate +2 WIS at between 0.5 feats for a non-WIS caster and 2.5 feats for a WIS caster.
+2 CHA: For a CHA-based spellcaster, it gives bonus spells and +1 spell save DC (better than Spell Focus).
So I would rate +2 CHA at between 0 feats for a non-CHA caster and 2 feats for a CHA caster.
I have omitted class features that depend on abilities. e.g. Paladins and Clerics get more bang for buck out of CHA due to Turn Undead, Lay On Hands, etc.
If you want to figure out the benefit after factoring in abilities affecting skills, just divide the number of skill dependent on that ability by 5 (comparing to Open Minded feat). e.g. If a Rogue has 5 DEX skills that he wants to improve, +2 DEX gives +1 to each, which is worth about a feat.
You should also keep in mind how easy it is to replicate a bonus with magic items. e.g. +10 to a skill is 10,000gp, +6 to an ability is 36,000gp ... so those are other benchmarks you could apply when estimating the value of skill points.
From the above analysis, I rate +2 to an ability in the following order:
At 1st level: CHA < WIS < INT < CON < STR < DEX, with no armor.
At 1st level: CHA < WIS < INT < CON < DEX < STR, with armor.
At 20th level: CHA < WIS < CON < STR < DEX < INT, with no armor. CHA < WIS < CON < DEX < STR < INT, with armor. Though this assumes that you want lots of skill points.
