I take some landscape photos, a lot of horse pictures and tons of DOGS! I have seen the most sales from Fotolia (especially for dogs and horses), but they only accept about 25% of my submitted images. FT seems to prefer West Coast images of the United States. I've also had success with Dreamstime, Shutterstock and Istock (definitely have more rejected than accepted at Istock - but the dogs and horses accepted DO sell).
Bigstock for me has been a DISSMAL failure with animal shots and landscapes (I've had plenty accepted -just no sales). For small companies I've sold a handful at 123rf, only a very small portfolio there though (less than 50).
Overall, my acceptance ratios are about 50%. I've been active since Aug. 2008 and I've made roughly $200. My biggest portfolio is on DT with 169 images, my smallest is at Istock which is less than 25.
Hope this helps.