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  to upload my work for maximum income?
Q: As a buyer, what are the best value stockphoto sites  to buy from?
Click on the above links for the answers to these important questions