Which online stockphoto-agency works best for photographers?

I am an artist who tries to find a way of generating some extra income. I do have a digital SLR photo-camera and a little photo-studio which I use to make photographs. I am considering to make photos of different subjects to sell them online through an online stockphoto agency, like bigstockphoto, dreamstime, fotolia, stockxpert, shutterstock, 123RF. Which agency (of these or others) has enough traffic and selling power to sell photographs this way? Which would anyone recommend? Any information is welcome.

shutterstock

where i work we frequently use getty, istockphoto, shutterstock, ablestock the most.

http://www.reflexstock.com/

This is quite a good site My friend actually works for this company and they are going from strength to strength.