I watched a youboob vid last night where a guy did a three tank wick test using organic cotton, rayon, hemp and one other kind of cotton(can't remember now what that one was)and the organic cotton and the rayon came up pretty much neck and neck as being the best. What made me choose the rayon was it showed less discoloration at the end of his test which makes me think it burned the least.
I just tried to find the vid again but no luck, maybe one of y'all know the one I'm talking about. I thought his test was unbiased and fairly scientific.