If used on a PC with 0 HP, Healing Word is the model of efficiency. Doesn't take up your whole turn to cast, gives someone a Move/Action/Bonus Action for just your Bonus Action. Does not get much more efficient than this. Also, efficiency measured on what scale? Number of encounters you can clear on a long rest? Least amount of spells cast? Number of hit points healed per day? The number of HP healed per day is the same for everyone, full health at the end of a long rest, and having any spells left when you do long rest is also inefficient.