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