The answer to this question is relatively easy, shutterstock [2] is the most 'popular', they have the largest amount of customers and largest image collection. Popularity is important, but the real questions you should ask are:-
Q: What are the recommended stock photo sites [3] to upload my work for maximum income?
or perhaps...
Q: As a buyer, what are the best value stockphoto sites [4] to buy from?
Click on the above links for the answers to these important questions
It's quiet in here!