Ammonia is generated by the deamination of proteins. The more protein you feed the more protein they poop so if you are thinking about ammonia load the protein eater produces the most.
Also as others have said - fiber is not the more easiest digested stuff - that is why certain animals have symbiotic bacteria in their gut to help them digest it. So fiber eaters may leave a lot of waste but not much of it as poop can be further degraded, in general.