Surprisingly, cottonwood produces almost as much heat per lb as any wood that there is. The problem is, you need big logs becuase the density of cottonwood is small--lots of internal air within. Many stoves haven't the interior volume to hold the logs because you need big cottonwood logs to crank out the heat.
Basswood (linden) and willow are in the same league. The big barrel stoves love cottonwood and crank out enormous heat with it:
The barrel stove on the right is used to burn cottonwood. The one on the left burns premium hardwood. The owner says the barrel stove is his bread and butter in January.