Most efficient method of tethering?
Your question has some ambiguity. Bluetooth is a more energy efficient protocol than wifi. USB hardly uses any energy for the data protocol.

However, with USB the laptop can charge the phone, as is mentioned here before. From the point of view of the phone, compared to bluetooth and wifi, the phone's power consumption with usb can be negative.

From the point of view of the laptop, the USB method can consume MOST power, if you include the charging current. This can be an issue if you run the laptop on battery power.

The total power consumption of laptop and phone is another issue, that you probably are not interested in.