There are a lot of power meters on the market and the first step to making a choice is to make sure any meter you are considering features direct power measurement. Strain gauges are the best way to measure power because you are directly measuring the deflection caused by applying force.
You also have to consider both the accuracy and consistency of your data, and understand the difference between them. An accurate power meter measures power within 1.5% of the actual power you are producing, but consistency is ultimately more important than absolute accuracy.
Even if your power meter is measuring 2% lower than your actual power, it is important that it measures 2% low today, tomorrow, next week, and next month. Consistency is ultimately more important than absolute accuracy because it enables you to monitor your training and track your progress with confidence.