I've gotta put in another vote for the nifftyseat.
Sure, it works best with a tree for a back rest but the advantage over tree seats that attach to the tree is that the size of the tree is irrelevant unless you get down into one inch or so sapplings that will move or shake if you lean against them suddenly. I have set up many times using that size tree and with a little awareness of my movent have had no problem.
For portablility, versatility and adaptability, there is nothing better that I've ever used, seen or heard of. The single leg design is a bit unique at first but it's advantages far outweigh any stability losses. Try setting up even a three legged stool on say a 45 degree slope and you will see what I mean. Try to put any multi legged stool up close to a very large tree with roots sloping away from the trunk and you usually have problems. The niffty seat will fit between two roots in a spot about 4 inches wide and will let you get close and use the trunk for a back rest. Folding chairs and such (in my experience) need to be set a foot or so away and then you lean back on the back legs to lean against the trunk. That's actually pretty comfortable until you need to shoot...
The hight settings also allow you to go from about 3 inches off the ground to (I think) a full 24 inches. It swivels dead quiet and is reasonably comfortable. It ain't a lazy boy recliner, but I've used mine for dark to dark sits when deer hunting and lived to tell about it. It does cause some discomfort and numbness after a couple hours but if you sat dead still in almost any normal chair for the same time you would have about the same amount of discomfort I think. The trick is to occasionally rotate the seat 90 degrees and go from sitting normal to straddling and back again as time and comfort dictates.