This is a long-time favourite of power meter enthusiasts, as the measurement is done in the rear wheel - mostly out of harm's way, and with plenty of room to do the measuring. Powertap sells the hub only, allowing you to build your choice of wheel. The wheel is obviously transferable between bikes, which is good. The "true" combined power of both legs is captured, but you won't be seeing any left-right balance data.

Current deals for the Powertap G3 hub

DT Swiss 460

Rear Hub

Manufacturer website: Powertap