The cord is being asked a lot to take the repeated stress of bending repeatedly in a circle - plus the longitudinal stress placed on it. The brittle ones won't take as much as the more pliable. 

Dog bane is one of the strongest and more pliable fibers available.

Yucca is very strong but more brittle.

Velvet leaf (Buttonweed) is OK for strength but also brittle-ish.

Hemp is a good standard - used forever as ropes, cords, bowstrings and more.

Nettle has silky fibers such as the dog bane but from my experience (all this input is from my own experience) tho the fibers of nettle are very strong, I find that they rot easily. Get 'em fresh and keep 'em dry in storage and you should be OK. 

Milkweed - I have heard and seen reference to this as being a fine material but I CANNOT extract the fiber from it here in KS. (and have not tried it elsewhere). Maybe it's a local thing? - so I cannot give input here.

I have used many fibers green to good avail in fire making - hemp, velvet leaf, yucca, basswood (real tree bark). Besides sometimes having too much moisture thereby causing slippage I have had success - sometimes better than when used dried. 

I base my success with how these work with the bow drill on the making of ONE fire only. I almost always use a new/different string from fire to fire as I don't want the hassle of losing the fire because of string breakage. In our video “Primitive Wilderness Skills, Applied” you see where we used the two strings we made for the fire separately and then corded together in 7-8 fire making attempts before we had success - green hemp. The chaff/bark was left on.