I'd say the hot iron. Blow-drying your hair can also be harmful; basically, extreme heat. It damages your hair and makes it frizzy and brittle... enough damage can even make your hair start to fall out.
Applying heat is always more damaging than applying chemicals on the surface only. Chemicals like bleaching your hair is also damaging, because it changes the chemical structure of your hair. But hairsprays and mousse doesn't. They're very mild chemicals, basically just like glue.
But heat practically "fries" your hair and takes out all its natural moisture and makes it really dry.