I'll throw a vote in for the Thermarest chair. I don't often use a chair, but when I do that's the one I use. As others have said you do need to be using a Thremarest pad to use this chair. A closed cell foam pad will also work ok in it.

The most comfortable chair I have used is my Bearikade bear canister. This solution is both heavy and expensive (if being carried just to function as a chair), but does double as an awesome food storage receptacle. I use mine only where canisters are required, but if I were to consider carrying a chair that weighs 1 1/2 lbs like others suggested then this is a viable option at right around 2 lbs as it would double as a place to store food. Heck, depending on your food storage system (mine weighs about 7 oz for a dry bag, cord and mini caribiner) using a canister might even have you bereaking even as far as weight is concerned over carrying a stool of some sort as you wouldn't need to carry the bear bagging paraphernalia.