CRD,
I use either Kevlar, Monocord, or 10 pound spiderwire braid (GSP).
All of them seem to work just fine, but I find that I cut less hair with the Monocord or Spiderwire. The straight Kevlar seems to slice right through if you aren't careful.
I avoid the Gudebrod 'rodwrapping' threads because they are too thick for me for this application.
Buddy