What is the worst type of wood to burn?
Unseasoned wood and wood that has high sap content are not advisable to burn in any sort of fireplace or wood stove.
As for the "types", it is kind of a hard thing to nail down, since all types of wood have representatives that are good and representatives that are bad.
Cottonwood is not very good stove wood. I have hear, that burning cottonwood has draw backs like, having to clean the chimney cap about once a month to prevent it from becoming obstructed by soot and creosote.