What is the cracker/popper
First thing you can say about it, is that this is the part that makes the whip crack sound. This is the part that breaks the sound barrier which means it has a lot of stress to endure, and because it has a lot to endure it will can be ruined pretty fast. It is much easier than to change the cheap cracker than expensive fall or braid. It's also very easy.
Second thing you can say about it is that depending on a material and thickness of the popper you can have different sounds. I for example, if I want to be much better with my neighbors I tie on the silent cracker at the end, and if I want to have very loud sound I tie on the suitable end.
What materials can I use for a cracker?
In my opinion the best are:
Nylon - this can make a very loud and "sharp" cracker. Usually you can have nylon as a thin thread for machine sewing, and crackers made from this are soft and loud if you make them properly. It's easier to make a sound using them than a polypropylene cord, but they can tangle them-self easier.
Polypropylene - I prefer this material as poppers are stiffer and have smaller tendency to self tying them-self during practicing. But because they are a little bit stiffer you need to have better technique or heavier whip for a nice sound. Also making technique is different as this material usually comes as premade twisted cord. Also there are few colors than nylon.
How to make crackers
How to tie a cracker to the fall