Lava rest between the styles of Earth- and Firebending. The other co-bending styles would be Combustion (Fire/Air), Snow (Air/Water) and Plant (Water/Earth)
definitely not waterbenders, because it’s not water. i’d say it’s more earth. lava is just hot earth and not flame.
I’d say lava would probably be best harnessed by a firebender and an earthbender teaming up.