Of the backpack mods I've played with, this is my favourite.

Modular Backpacks is the most elegant implementation of backpacks I've seen yet.

It focuses on horizontal progression, adding features to carry with you,
while avoiding the pit-fall of making vanilla features obsolete,
cough sophisticated backpacks cough.

It also, funnily enough, adds the least inventory space, giving you only an extra 9 slots;
with a maximum of 81 slots with all the upgraded pouch modules.

I personally quite appreciate this,
as it helps me steer away from a pure horder life-style that I often succomb to
with backpack mods that give you quadruple chests worth of space.

There is one minor issue, that of you not being able to take off the backpack or any of it's modules without fully emptying both, which can be rather annoying at times, especially with the extra-large pouches which fill up half of your inventory on their own. An option to simply place a backpack on the ground would be greatly appreciated.

There's even cute costumization options!
You can hang trinkets on the buckle! How cute!