Not only is Apple the largest tech company in the world, it's also the greenest. For the third year in a row, Greenpeace has ranked the iPhone maker number one on a list of the most environmentally friendly firms.

In the agency's "Clicking Clean" report, Apple was awarded a final "A" grade and a clean energy index score of 83 percent. The company received top marks for energy transparency, renewable energy commitment, energy efficiency and renewable procurement. The only category in which it didn't get an A was advocacy, where it was awarded a B.

Two other tech giants received the highest grade - Facebook and Google - which had clean energy indexes of 67 percent and 56 percent, respectively. Nevada-based data center company Switch also received an A.

The report states that Apple has been an influence on other firms when it comes to clean energy use. The company "played a catalytic role within its IT supply chain, pushing other IT data center and cloud operators who help deliver pieces of Apple's corner of the internet to follow their lead in powering their operations with renewable energy."

Microsoft and Salesforce received Bs in the report, while Amazon only managed a C. Jeff Bezos's company may have committed to building solar farms and wind farms to provide power for AWS, but the firm's lack of transparency - a category where it got an F - cost it a higher overall grade. The "lack of transparency and its rapid growth in Virginia and other markets largely served by dirty energy, it remains unclear whether the AWS cloud is actually on a path to becoming renewably powered."

Bottom of the list was Tencent, which got Fs in all but one category.

The report states that the IT sector now accounts for 7 percent of the world's electricity, a figure that is expected to rise over the coming years. But Greenpeace notes that a number of companies on the list have committed to 100 percent renewable energy.