I use only one type of tying thread, called flat waxed mono 210. Danville and UTC among others make it. This thread has been my choice for a couple of years now. I have tried others but this is always my favorite.
Gel spun is strong, but cuts through the deer hair instead of compacting it. I also have trouble with the heads wanting to spin on the hook shank.
Kevlar and others are good when spinning hair, but they don't stretch and I have trouble keeping even tension on my flies.
There is another called Big fly or something like that. Larry Dalberg talked about it at the show last week, but I have not tried it yet.