MOLLE and his PALS

I really like MOLLE system. It allows more efficient use of storage by letting you wear multiple pockets on the same layer without conflicts. Not only that, but the overall encumbrance is also reduced because when an item is attached to a carrier, only the additional encumbrance of it's storage volume is accounted for - the baseline encumbrance when empty is ignored.
An extreme example of this is the front-to-back double holster: it has a whopping 8 encumbrance when empty, going to 10 when both pockets are full. When attached to a load bearing vest, however, it only increases it's maximum encumbrance by 2, with no effect if it is empty.
The regular holster causes 3 encumbrance when worn and it goes up to 4 with something in it - if it is attached instead, it will add nothing when empty and +1 when filled. Using it as an attachment also has the extra benefit of not occupying a layer on your leg that you could use to carry a flask full of vodka or a scabbard with a claymore.
As an additional note, the holster has an XS variation that has triple encumbrance penalty if your character isn't tiny, this penalty is also ignored if you attach it to something instead of wearing it directly. It is the only attachable item with a XS version.

Now that you understand why you'd want to use it, you need to understand how to use it. Clothing with MOLLE capabilities have it listed on it's actions as "Attach pockets" and "Remove pockets", they have a certain number of slots for that. Some of them also have MOLLE right there in the name.
Containers with straps to attach to MOLLE will state that they can "attach directly to load bearing vests", each item will take a certain number of slots when attached.
To attach a pocket, simply activate the MOLLE item while having a valid container nearby or in your inventory.
Figuring out how to remove an attachment is left as an exercise for the reader.

With the why and how figured out, the only thing left is the what. Specifically what attachs to what.
MOLLE can commonly be found on dead soldiers and cops, and can be spotted by the "+3" (or some other number) on their name, indicating how many PALS are attached to them. Feel free to remove the perfectly clean PALS in pristine condition from the filthy ripped body armor, and then attach to something else.
Here is the full list of items to look out for:

Notes:

• Upon attaching a PALS, the MOLLE will gain the BELTED flag on the torso. You will only notice this on a MOLLE that covers the torso, but is not strapped to it - i.e. only body armor (ballistic vests and riot armor). Attaching to them will make them conflict with other clothing strapped to your torso. EOD trousers do not cover the torso and are not affected.
• Some items did not have a defined "volume_encumber_modifier" field in their JSON definitions and it defaulted to 1. I tested and can confirm that they indeed get +1 max encumbrance for every 250mL of max capacity added.

And here are all the attachments, alongside possible values for their max encumbrance depending on where they are attached:

Notes:

• I did not include the VEM in PALS because it gets overwritten by the MOLLE's VEM when attached.
• Holsters have a nylon version. The only difference is a minor (+250g) increase in their weight capacity. This doesn't matter for most use cases. It also has less armor than the leather version, but again, this shouldn't be taken into account unless you're crazy. Though the nylon version has slightly lower weight (the item itself, not the pocket) so this might be worth considering if you're a bird mutant or something with extremely low carry capacity.
• In the case of the chest ammo pouch, the leather version is considerably heavier than the other, but they are otherwise identical. Ditto for the ammo satchel and stone pouch.

Now you must be asking yourself what is that VEM thing. But for that to make sense, I need to explain a bit of the pocket system first.

There are two types of containers: rigid and non-rigid. Rigid containers always have the same size and thus the same encumbrance. Non-rigid containers have a minimum volume when empty and will grow as it gets full.
The encumbrance caused by a container you wear depends only on its volume; weight doesn't matter. That's why rigid containers have a static value: since its volume doesn't change, it simply uses its defined encumbrance without changes. Non-rigid containers will linearly increase their encumbrance as their volume grows, up to its maximum defined value when at maximum capacity.
If the maximum value is not defined, it will fallback to VEM; it stands for Volume Encumber Modifier, defined in the volume_encumber_modifier JSON field.

There are 2 ways in which the range of encumbrance of a non-rigid container is defined: either as an actual range in the form [empty encumb,full encumb] (the standard holster is defined as [3,4]), or by specifying a volume_encumber_modifier field and letting the engine figure out the additional encumbrance based on that, according to the formula: VEM * stored volume / 250ml
The default VEM is 1, meaning that the container will gain 1 extra encumbrance for every 250ml it holds. That's the baseline for something that's terrible at distributing weight - proper wearable containers have much lower VEM. Lower VEM is good.

When it comes to PALS, they all have a defined VEM of 0.3 (except for the canteen pouch, it has 0.2), but it doesn't matter because you shouldn't be wearing them directly anyway. When attached to a MOLLE, their VEM gets replaced by the MOLLE's - therefore the same setup on different clothings will have different max encumb.
More encumbrance-efficient MOLLEs probably have a higher baseline encumbrance when empty, so they aren't worth using unless you're stacking them full of PALS.
Here is a graph comparing the encumbrance on load bearing vests according on their volume:

My Hot Takes

On MOLLE

Don't use the ones with undefined VEM.
Also don't use the ones in the Other section.

• The MOLLE webbing belt has the worst VEM out of all items. You're better off with a light LBV, it has the same number of slots anyway. The belt is only better if you already have something else strapped to your torso.
• Riot armor is bad against bullets and even for melee it has bad coverage.
• The mercenary coat has the same base encumbrance as the ballistic vests, but lacks ablative pockets. Its only upside is full coverage of the arms, but if you intend to stay alive then it's much better to have ceramic plates on your torso than covered arms. You can compensate by using a heavy ballistic vest with should protectors and wear vambraces plus elbow pads. But even if you don't care about bullets, there are better options for armor in terms of overall encumbrance and bash/cut/pierce defense.

I don't know why you would want MOLLE backpacks, but...

• The assault pack (the most efficient one) is still worse than the regular backpack, it even has lower capacity (20L vs 25L).
• If you really need MOLLE slots on your backpack, the assault pack is the best one. All packs have the same VEM of .15, but the assault pack has the least encumbrance overhead. Remember that any attached pockets will also inherit the .15 VEM, so if you need more capacity, its better to attach gas mask pouches than to switch to the medium rucksack.
• If you still need more space, the medium rucksack has twice the space (+20L) with only +1 encumbrance overhead. The change from medium to large isn't as good, however. You only get +10L at the cost +2 base encumb (not counting the additional max encumb from the extra space). You do get +5 max length, though, maybe this detail might be relevant for your case.
• The hunting backpack has the worst efficiency, but also the highest capacity (72L on the main pocket, up to 80cm). Personally, I'd only use it after I'm done with a raid and ready to steal everything, including what's nailed down.

I already compared the Load Bearing Vests, so now I'll talk about the Ballistic Vests:

• Don't use the light BV unless that's your only option. There are 3 variations of them: (mag pouches), (shoulder pouch) and (storage pouch). The only difference is the type of built-in pocket they have. (mag pouches) is the worst one, (storage pouch) is the best one.
• The only downside of the light BV over the light LBV is that it also occupies the outer layer, so you can't wear it over armor without penalties. But if you don't want to wear anything else on the outer layer, you should use the light BV for storage instead of the light LBV. It has the same encumb, more slots, and actually provides some defense. Bear in mind that it only covers the upper torso, though.
• The heavy BV is better than the regular US/SWAT vest. It has slightly better lower torso coverage (90% vs 80%) and accepts protectors for other parts. It is a bit heavier, but has the same base encumbrance.
• If you're wearing a BV, its best to attach your stuff directly to it instead of wearing a LBV over it. It has the same VEM as the light LBV (so you probably won't want both together) and in principle the medium and heavy LBVs would be more efficient at higher capacities, but due to the overhead, the heavy LBV won't be better unless you wanna have more than 10.5L capacity. And the medium LBV is never the best option at any point (see graph).

On PALS

The ones that carry only specific items should be used only if you use that specific item.
Only the spiciest of hot takes on this site. ((and I say "only" a lot))
But you should probably carry magazines instead of loose ammo, unless you're using a revolver or something.
As for the ones that accept exclusively magazines, you should stick to the chest ammo pouch. It can fit a good handful of mags, and by process of elimination: the triple stacker and ammo satchel are overkill, ankle ammo pouch will probably be too small for you, and the leg ammo pouch is worse than attaching 2 chest pouches.
Oh, and both pockets for grenades are bad, just stash your nades in a general purpose pocket. Same for any loose ammo you might wanna carry outside of mags.

We can also eliminate useless holsters:

• Side-by-side is worse than 2 tactical.
• Front-to-back is worse than regular+tactical.
• You should only use a large holster if you have a gun that wouldn't fit in a regular one.
• Go ahead and use the fast-draw holster if your gun can fit in that (it probably can't).
• I think it only makes sense to use the tactical holster if you're short on slots, but the +20 movecost is pretty minor so it doesn't really matter.
• The canteen pouch is a great choice for any quick-access item.
• If your sidearm is <=23cm then the canteen pouch is unironically better than a real holster.
• I saw other people talk shit about the flashlight pouch when they compare PALS, saying that it is categorically worse than the radio pouch (same capacity and movecost, but can fit multiple items). It is true to some extent, but consider this: radio pouch can fit a maximum length of 13cm while the flashlight pouch can fit 37cm. I like to keep a clawbar in there to pry doors and nails until I get a hooligan bar, which I then keep in a large belt loop.
• The use cases for the belt loops and sheath are pretty clear. Use them if you want to carry an item that fits in.

The general purpose pockets are a bit harder to rule out and will depend on what you with to store, but here is a rule of thumb: you don't want a bunch of useless clutter slowing you down in the middle of a fight. It's good to have first aid supplies on you for when you inevitably take a serious hit, but I prefer to use an armband pouch for that instead. Seriously, it can fit a small homemade grenade and still have space for bandages, alcohol wipes, painkillers, and homeostatic powders - all at the cost of zero encumbrance. On the armband on other arm I like to put small tools, a cellphone and a small scope (for better mapping).
In short, if you want to use your body armor for storage: don't.

I think the only real application of this type of PALS is to increase the capacity of backpacks. To that end, stacking a bunch of gas mask pouches give you the most bang for your buck, with 2.325L per slot. The deployment bag comes very close, at 2.13L per slot, and might be a better choice if you need to carry denser items.
If you somehow need to store multiple long items, you can use the tactical tool pouch - the only other containers with higher max length are single-item.
Special mention to the tactical radio pouch, by the way, for having a movecost of 40; it is practically a multi-item holster. (if you can find something that fits in there)

Making a Loadout

Now that you know all there is to know, here are the basics to build an efficient loadout with MOLLE:

• Don't bother with a backpack. You will be dropping it during combat anyway, so whatever it is that you want to carry in one, it won't be vital enough to warrant worrying about it.
• If you are already wearing a ballistic vest (preferably a heavy one, full of ablative armor) you have no reason to wear a LBV or MOLLE belt. I only recommend them if you're wearing some other type of outer armor. And I'd only wear the belt if the armor is also strapped, like the Hub01 armor.
• Then wear a single point sling over it, plus a back holster. These are to carry your main melee and gun. The back holster is optional, you can wear a long gun on your back instead. Leave the backpack for when it is safe to stroll around.

And here is the stuff you're gonna attach:

Alternatively...

Of course, that's assuming you want to wear a BV as your outer armor, but you might not fear bullets and prefer to wear a suit of armor or something like that instead. In that case, go ahead and put the above in a LBV. Hell, attach a sling to it, even. Just replace the holster with a tactical holster so it all fits in the 8 slots of a medium LBV.
The LBV might be slightly less efficient, but it has the added benefit of easily being worn over anything else.