Diane,

I'm pretty sure that a couple of guys I know who tie the tightest packed spun deer hair flies use GSP thread. Probably a lot has to do with the size of the bunch of hair used. On smaller flies you can get by with 6/0 or 3/0 or monocord. On flies where you'll typically use larger bunches, you may need stronger thread. I've always read to stay away from kevlar.

Hope this helps.

Allan