One of the most common questions we receive from customers during the purchase process of a power meter is which system is "best." Power may be measured at the hub, crank arm, pedal, crank spider, bottom bracket, etc.. There have even been systems that measure at the chain or extrapolate it using a bar mounted sensor. The first point to make here is that there is no "best" place but moreover what is best for the customer; and there are some key questions you should ask yourself before purchasing.
When power meters were first introduced they were limited to a very few key brands, namely Powertap and SRM. Both were expensive and rather finicky. Powertap measured power at the hub, and SRM within the bottom bracket. Ironically both these brands, once the leaders in their market, have been rendered mainly obsolete. I recall a rep telling me emphatically that power simply could not be measured accurately in a pedal based system. I also recall another rep telling me that power could not be measured accurately using a sensor on a crank arm. Both of these reps, and the companies they represented turned out to be dead wrong. They failed to adapt with emerging technologies while other companies such as Garmin, Stages, Pioneer, and Quarq ran with them eventually producing products that were more simple, flexible, easy to install, and cheaper.
The drawback of a hub based system (Powertap) was that you had to either purchase one of their stock wheels or have it built into a wheel of your choice. This limited selection and increased cost. For those that had a training wheelset and an expensive carbon race wheelset this meant purchasing two hubs or wheels to meet their needs. It also meant increasing the weight and/or durability (warranty) of their race wheels by integrating a Powertap hub. From the get go this was a potential liability in purchasing a Powertap. An SRM, being bottom bracket based, did not affect wheel selection or performance but was significantly more expensive, required the use of their head unit, and also required disassembly and shipping of your SRM back to the factory for a simple battery change.
Garmin was the first to introduce a pedal based power meter in the Vector but it was mainly a disaster. Later Vector models finally became reliable enough to recommend. Quarq leapt ahead of SRM with a crank spider based system that was accurate and used a simple coin cell replacement battery. Powertap finally realized hubs were on their way out and introduced the P1 pedal based system, a bit more clunky than the Vector but functional, as well as the C1 chainring which did not do well enough to continue production. Powertap was eventually purchased by Quarq and some models remain available. Stages and Pioneer introduced the non-drive crank arm based system at a lower price. I am mentioning only the major players, there were and are a lot of other ancillary brands but Stages has emerged as the world leader. Stages has consistently improved their products making them smaller, more durable, more accurate, but most importantly lowering their prices and pressuring other brands to follow. This strategy has worked very, very well for them. Pioneer recently announced they were folding their tents.
This brief history of the power meter demonstrates foremost that those that say it can't be done are usually passed by companies doing it, and the consumer has benefit the most. Power meters are now mainly either crank based at the crank arm or spider, or pedal based. I only recommend a pedal based system if the customer wants the flexibility of easily moving their power meter from bicycle to bicycle using the same pedal system. An example would be moving their pedals from their road bike to tri bike. This is also a system that can actually be traveled with. For most others replacing a single crank arm or having a power meter factory installed on their existing crank arm (another new option) is going to be best. I only move toward replacing an entire crank if they are interested in dual power measurement (left/right). This is of dubious value for most cyclists, but has merit for some. My prediction is that power meters will continue to become cheaper, smaller, and more integrated. When purchasing a new bike, or crankset, a power meter will be an option or simply included in the sale price to make the product more attractive. The technology, like most technologies, will go from being expensive and exclusive to cheap and commonplace.