The most comfortable hammock is the canvas. But in reality, the subject matter: the real comfort is to feel in a cocoon, suspended without the air, and to enjoy nature without touching the ground!
Below are some terms you should follow while choosing the best hammock or you can visit the hammock forum to find more details.
Hammocks 100% cotton
100% cotton hammocks are the most comfortable. Nevertheless, they are rather heavy, not obvious to transport and dry (you must expand, it may be easier in a garden but less so if you decide to install a hammock in your bedroom).
Most brands use cotton called air trade in a country in South America.
The organic cotton is much less common, mainly because it is more interesting because of (economically and environmentally) using recycled fibers, and also that certification has a cost, which requires economies of scale and thus of large farms.