I struggled for years trying to make cattail rope, because it would often dry brittle. But I've learned to be more careful, and have since made many nice big pieces of cattail rope. The biggest advantage is that you can make a lot fast. I like to double reverse wrap it, which usually takes care of any weakness at spliced parts.
Otherwise my favorite material is milkweed. Not quite as strong as dogbane, but close, and we have milkweed in abundance here. Plus, I like that it's a bit sticky. The fibers end up sticking to each other.
Nettles are great too, like Peter said. I'm actually just about to process some that I gathered a few days ago.