I think with all the negative opinions about iStockPhoto running around, it is important to note that they are still my top earner (with Shutterstock). It is trully a pain to upload images. It takes me 5-10 times as long to complete a submission than on Fotolia and about 30 times as long as a Shutterstock submission.

But they bring in the money.

I think that "best" is obviously very subjective and depends greatly on your portfolio. However, they still bring in a large percentage of my income.

In the last two months I have decided to try out something else. Instead of submitting new images to iStock, I am taking that estimated time to shoot 10-15 image a week and submit them to the other majot agencies. It is to early to tell and I'm not sure what I will have to report other than an estimate, but I am thinking I will earn more total this way.