What is the most efficient amplifier?

When it comes to amplifiers, there are countless options on the market, each with its own unique features and capabilities. But, what truly sets one apart as the most efficient? Are we looking for an amplifier that minimizes power consumption while still delivering exceptional sound quality? Or is it one that maximizes gain while minimizing distortion? Perhaps it's a combination of both? As a discerning consumer, I'm curious to know which amplifier stands out as the most efficient, balancing performance, efficiency, and cost-effectiveness. Could you please elaborate on the characteristics that define an amplifier as highly efficient, and perhaps provide an example or two of models that embody these qualities?