The thermarest chairs and their various cheaper knockoffs are about the most comfortable because of the back support. I have used lightweight three-leg stools in the past but they don't have back support (which is solved if there is a nearby tree) and you have to lean to do things like cook if your utensils are on the ground.
The chair converters for inflatable mattresses are extremely useful if you have to hole up in a tent for any length of time because they are not likely to damage the floor.