The thickest ABDL diaper is likely the Rearz diapers although I never tried them.
The thickest ABDL diaper I personally have tried and have worn and do wear is the ABU Super Dry Kids.
Bambino was the thickest diaper I first tried before I found ABU Super Dry Kids.
Although not an ABDL diaper the Northshore Mega Max adult diaper is also a very thick and very absorbent diaper that many ABDL people love to wear.
Some of the best ABDL diapers I've tried are ABU Little Pawz, ABU Super Dry Kids, Bambino Diapers.
I haven't tried Rearz diapers but I hear they are good thick and absorbent diapers and I hope to try them soon.
Bambino diapers were the first ABDL diapers I tried and I love them and I still love them and wear them sometimes but mostly I wear ABU Super Dry Kids diapers as they resemble the old style thick pampers.
Although not an ABDL diaper the Northshore mega max diapers are also good thick and absorbent diapers.