Paizo is wrong about item slots
They don’t matter, and trying to force them to matter makes the game worse
Paizo maintains that item slots are very important, that they must be, as the question puts it, “heavily regulated.”
They could not possibly be more wrong. I have a vast amount of experience with Pathfinder and its antecedent, and I have seen this detail come up in myriad ways countless times—and item slots add nothing of use or value to the game. I have basically tossed them out of my games wholesale, and they are vastly better off for it.
Item slots rarely make any kind of difference
First of all, with one glaring exception—the belt you note, which we’ll get to—item slots barely come up at all. Anything that makes one slot more valuable than another is coincidence more than anything else. In fact, the one slot that’s most notable in my mind is actually the ring slot—but you have two of those, which compensates. The balancing factor for magic items is their cost, not its slot. Slots are chosen by authors based on “feel” for the most part, and have minimal impact on balance.
Authors don’t balance around item slots
That’s because the authors don’t really balance around item slots. As an occasional third-party Pathfinder designer and author, I can assure you that authors do not go through the list of magic items to see what the item they’re writing would or wouldn’t conflict with in each slot. That would be a monumental amount of work, and would that’s after we make a number of simplifying assumptions about the character who might want to use the item and what other items they might like to use. Paizo’s release schedule—and pay structure—does not remotely support that level of effort going into each magic item. Authors just go with their gut about what feels right for the kind of item they’re working with; it isn’t about balance.
Wizards of the Coast figured this out, and eliminated most of the problems
As a player and GM, I can also tell you this more-or-less mostly works out, but when it doesn’t, it’s not a matter of balance, but as I said, coincidence. This is an arbitrary frustration the game occasionally throws at you—and a frustration is all it is. And you don’t have to take my word for it—Wizards of the Coast, who published the D&D “v.3.5 revised edition” that Pathfinder is based on, had this to say about item slots in their Magic Item Compendium:
One of the most frustrating roadblocks to using interesting, unusual magic items is that they take up body slots that you need for an ability-boosting item (such as gauntlets of ogre power), a ring of protection, or another must-have item. To address this issue, Magic Item Compendium presents official rules for adding common item effects to existing items.
(Magic Item Compendium, pg. 233)
There it is: “frustrating roadblock.” That’s what magic item slots amount to.
This paragraph is the introduction to a new rule offered in Magic Item Compendium wherein “common” magic item effects—resistance bonuses to saves, enhancement bonuses to ability scores, energy resistances, and various bonuses to AC—were allowed to be freely (as in, with no extra surcharge) added to other magic items. So your heavyload belt could also be a belt of incredible dexterity and it wouldn’t cost anything extra, just the cost of the heavyload belt plus the cost of the belt of incredible dexterity.
The same chapter also indicated numerous potential items slots that these common bonuses could appear on, and indeed, in D&D 3.5e, the default source of an enhancement bonus to Dexterity were gloves of dexterity, not a belt. Strength bonuses even appeared as both gauntlets of ogre power and belt of giant strength right in the Player’s Handbook (core rulebook for players). Despite otherwise having nearly identical rules to Pathfinder, D&D 3.5e did not make this issue about ability score bonuses such a big deal.
Is Pathfinder different in this regard from 3.5e? No, if anything it’s worse.
So, Paizo intentionally chose to change things from the D&D 3.5e core rules to make ability score bonuses harder to get more than one of. Did this improve the game? No, it objectively and emphatically made things worse. In contrast, Wizards of the Coast changed things, in their later publications, to make it easier to gain these bonuses—and that made things better. Again, I have played many, many games under each of these rulesets, and there really is no competition here. I have personally gone even further than Wizards of the Coast did in Magic Item Compendium, and that made things even better.
To see why Paizo’s approach is so terrible—and Wizards of the Coast’s is superior—we have to look at the one item slot where this kind of thing comes up again and again and again in Pathfinder: the belt. Paizo changed all the items that granted enhancement bonuses to physical ability scores to use the belt slot, which wasn’t the case in 3.5e (in the core rules, Strength could use gloves or belt, Dexterity used gloves, and Constitution used an amulet). They also published guidance for custom magic items that called out GMs who considered changing this state of affairs, telling them it was bad/wrong/dangerous (in contrast to Wizards of the Coast, who eliminated the extra costs for combining these with other magic items altogether). And these changes mean basically one thing: you are penalized if you need to improve more than one physical ability score.
This is a huge problem, because everyone more-or-less needs bonuses to Constitution. As I was just explaining in another answer, a Constitution below 14 is nearly suicidal in Pathfinder—and at higher levels, your needs go up from there. For spellcasters, this is no problem: they can have their headband enhancing the mental ability score they need—and with very few exceptions, they only need one (the one that sets their save DCs and bonus spell slots)—and then they can have a belt enhancing their Constitution. But for physical characters—who absolutely need enhancements to Strength or Dexterity (if not both) if they’re going to hit anything, they have a conflict. A conflict that is only resolved by buying a belt of physical might, which costs substantially more than a belt and a headband cost as separate items. The physical character needs to waste more money, and needs to save up longer before they can upgrade, as compared to a spellcaster. They are, in very real ways, heavily disadvantaged as far as items are concerned.
So, does it make sense that warriors would have a handicap here? Are they more powerful, need to be taken down a couple of pegs to balance things out. No. The warriors are already, independent of magic items, the weaker classes in the game. Magic dominates everything in Pathfinder. “I have a higher-level spell than you do,” is very close to saying “I have already won this encounter.” This is reflected in the very-widely-accepted tier system, which categorizes each class based on its relative power and flexibility. All the top classes are the pure spellcasters—cleric and wizard and so on—and the lowest classes are all mundane warriors and skill monkeys (and the “magical-but-not-spellcasting” classes like monk and kineticist).
In fact, if anything, relative to D&D 3.5e, Pathfinder is arguably worse, and certainly not better, about balance between the mundane and the magical. So making it harder to get multiple physical ability score enhancements makes even less sense in Pathfinder than it would have in 3.5e.
Paizo figured it out too—for Pathfinder 2e
Pathfinder 2e doesn’t have item slots. It’s a very different game, but on some level, it shows that Paizo realized exactly what I’m saying here: it doesn’t work out as a meaningful balance mechanism, and winds up being nothing more than a “frustrating roadblock.” So they ditched them—magic items in PF2 are balanced in other ways. And they’re balanced in other ways in PF1, too!—their costs. The gold cost is the primary balancing mechanism on magic items.
Conclusion and Recommendations
So we have a system which is complicated and arbitrary, that when it does matter, mostly shafts the weakest classes in the game. It adds nothing, and should be jettisoned at the earliest opportunity. I heavily recommend the Magic Item Compendium rules, if not going further and applying them to all magic items—just allow magic item effects to be combined however the crafter (or the PCs paying them) want, with no extra surcharge. If they want a cape with all the magic on it, fine. If they want boots that improve their Dexterity, why not? It does not matter—and/or, when it does, it does so stupidly.
But if you cannot or will not do that, then the answer to your question is that it’s arbitrary and largely a matter of coincidence which item slot is most important; there is no over-arching valuation of each slot that is consistent and objective and utilized by designers for balancing magic items. They are all roughly equal in value, except sometimes to you, personally, because of your specific needs, in a way that can’t be generalized. The only sort-of exception is the belt slot itself, so a GM who really bought into Paizo’s nonsense on this subject and really felt it was important to screw over the weakest classes in the game “should” probably tell you it has to be a belt, because that would be very consistent with the awful suggestions that Paizo has made.