A true ultralight enthusiast would be sleeping on a bed of dander and leaves in the forest.
However, I like a good nights sleep, and this thermarest does it for me.
Granted, a little bit of a luxury at a whopping 13.1 oz. Get the short 3/4 length model – you really don't need anything longer – just drop your pack at the bottom for your feet to rest on.
Available from Amazon for $67.