Depends what grows where you are. Hybrid willow and poplar will give the most volume of wood but it needs to be dried and kept dry. Alder is a fantastic and under-rated firewood. Fast growing and easy to split plus it coppices readily. You might want to also consider ash, hazel, sycamore and others for longer term coppice hardwoods. A mix is probably best with a range of harvesting schedules.
Have a look at what's growing on the land already and explore the potential of those species.