The Extra Fine Poly Dubbing from Fly-Rite is made of polypropylene (on wikipedia if you want to read up some more on this) and is very good for slender bodies and dry-flies. The material is very easy to work with and goes on the thread with no problems. For now I only have this in golden yellow, but would like to get this in a few more colors to work with on emergers.
The individual fibers are very long:
It goes on the thread (here a Sheer 14/0) very easy and does not build up to something big and bulky
This is a Mustad 44840 #14, and I have tried to build up as small as possible: the body gets very slim and is just thicker than the hook.
Here you can see the reason why I like this on small flies: here on a Gamakatsu C12-BM #30, without much effort you get a body on even these small hooks