Which is the VERY best agency yo start with? I am a 10-year wedding photography trying to do something else. I think I have about 200 images to upload right away, not sure how good they are.
Shutterstock seems to give the greatest return to the majority of the forum users as far as i can tell, followed by iStock, then Dreamstime, Fotolia and Stockxpert.
That is the usual order for my sales numbers. I am keeping a record of my experience with these sites at my blog: http://bcstockphotos.blogspot.com if you're interested. I'll be updating it a bit more soon, but it has some figures for the last year or so.
Where you should sell depends a lot on the amount of time on your hands. I would recommend starting with the first three or four agencies on the photographers microstock ranking list. From there its a matter of preference, the top 5-7 agencies will make some money, the rest are just getting started.
Many people seem to like LuckyOliver even though they are still small. It will be interesting to see how it turns out.