No, I agree, nettle is excellent and the most widely available.
Willow is wonderful if taken at the right time, and even at the wrong time, when the bark comes off in little bits, it still makes really good rope, it just needs more preparation.
Most of the bulbs produce good fibre rich ropes from their wilted tops, daffodil, crocus, iris, gladiolii....all strong and lasting too. The little lobelia is actually a member of the flax family and makes good rope as well. Flax is superb and so is hemp. Elm and lime bast are brilliant, chestnut is good, so is honeysuckle and bramble and briar.
Even grass makes a good rope and the leaves from oats, wheat, barley and the cat tails make fine string. Rush rope is used for basketry and the little field rush stripped down and twisted and laid makes a really strong, neat string. Heather makes a coarse but long lasting rope that survives well in water.
cheers,
Toddy