Probably not much comes close in terms of volume of wood but willow is never going to be the best firewood, especially on an open fire. All dry wood pretty much burns and gives out energy equivalent to it's weight more or less. So, a cube of willow is going to weigh much less than a cube of oak and therefore give you less heat overall as it will burn faster due to the lower density. If you have a good stove with good air control, willow will be as good as anything apart from maybe need re-fueling a little more often. You must also keep in mind that lighter woods will season far more quickly than dense woods so that's an advantage.
But growing oak for firewood is not really a goer unless you are planting for your grand-children and harvesting the stuff your grandad planted! It's also more valuable as a timber crop anyway.