I really like DandY and MDA.
You can also look at 2 sites"
Most of the agencies do a reasonably good job. One danger sign to look out for is when you get handed off to multiple people. I had the best experiences when I had one contact at the agency.
If you do locums, be sure to call ahead to the hospital and ask about the place you will be staying. Some hospitals are in crummy areas and the agency puts you up in the place closest, which may not be good. You could be better off staying further away.