Which organism would you most likely find in the most extreme habitats on earth?

The organisms that would be most likely found in the most extreme habitats on earth are the Archaea. Archaea is any group of single-celled prokaryotic organisms; that have distinct molecular characteristics separating them from bacteria as well as from the eukaryotes. they have features such as; their cell walls lack peptidoglycan, their membranes have ether bonds connecting fatty acids to molecules of glycerol.