Depends on the bow, how the string is made, number of strands, how the nock fits, silencer type and placement, etc. etc. etc.
My personal favorite is Dynaflight '97. I get lots of comments about how quiet it is. I've had people tell me that their recurve (with Dynaflight) was as quiet or quieter than their longbow. I've also heard different people that prefer 450+, 8125, etc. I've seen some posts about TS+, but I've never tried it. I'm stuck on the customer service BCY offers, so that's the people I deal with.
Some bows seem to "like" one material over another, but generally speaking I think the other variables (tuning, how the string is made, silencer placement, nock fit, etc.) makes a lot more difference than the string material--at least it has in my experience.
That being said, I never did care for regular FF, but I never tinkered with it either.
Chad