I frequently tell people that the first thing I do when I get a new pocket diaper is ditch the microfiber insert. In addition to being prone to odor issues, they just don't seem as absorbent as other options. I prefer AMP bamboo or infant prefolds for daytime, and AMP hemp inserts for overnight use. Here's a list of some common diaper inserts and how they compare*:
Bummis Organic Cotton Prefold (toddler): 319ml
AMP Hemp insert: 275ml
Bummis Organic Cotton Prefold (infant): 217ml
AMP Bamboo insert: 187ml
Osocozy Unbleached Cotton Flat Diaper: 165ml
Thirsties Hemp Insert (large): 150ml
FuzziBunz Microfiber Insert (lg): 110ml
FuzziBunz Microfiber Insert (sm): 105ml
Bummis Organic Cotton Prefold (newborn/preemie): 103ml
Price-wise, they stack up a little differently:
AMP Hemp insert: $8.00/ea
AMP Bamboo insert: $8.00/ea
Bummis Organic Cotton Prefold (toddler): $5.00/ea ($30.00 / 6pk)
Thirsties Hemp Insert (large): $4.00/ea ($8.00 / 2pk)
FuzziBunz Microfiber Insert (lg): $4.00/ea
FuzziBunz Microfiber Insert (sm): $4.00/ea
Bummis Organic Cotton Prefold (infant): $3.17/ea ($19 / 6pk)
Osocozy Unbleached Cotton Flat Diaper: $2.67/ea ($16/ 6pk)
ummis Organic Cotton Prefold (newborn/preemie): $1.83/ea ($11.00 / 6pk)
Pocket diapers are all shaped a little differently, so not all inserts will work in all diapers. FuzziBunz and Charlie Banana one-size diapers are very narrow through the gusset -- AMP bamboo and Osocozy flats fits these best. AMP hemp can be a bit rigid for daytime wear, so are best at night in an AMP or BumGenius one-size diaper. You are only limited by the width of your diapers and your imagination.
Ease of washing is another consideration. Osocozy flats are one layer and can be folded to fit. AMP bamboo and hemp are 2 and 3 layers and can also be folded to fit. The fewer the layers, the better inserts will wash and dry, and the better they can be folded to fit your particular diapers. Osocozy flats are especially nice as they line-dry in the same amount of time as the shells!
Hopefully, this list will help you get the most absorbent bang for your buck. Do you have a favorite insert I haven't listed?
*Clean, dry insert thoroughly soaked in water and then allowed to hang for 40min, until no more water dripped from fabric. All inserts purchased myself for use with Boy#3.