"Listless" already implies the highest degree of being lacking in energy or enthusiasm. To convey a stronger sense of this, you might use intensifiers like:
* Utterly listless
* Completely listless
* Totally listless
* Exceedingly listless
* Remarkably listless
You could also use more descriptive words like:
* Apathetic
* Lethargic
* Enervated
* Prostrate
The best choice will depend on the specific context and the level of intensity you want to convey.