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.