I don't think anyone is going to rank them, since that would be as hard as trying to tell people what antivirus is best.
Besides what is best is subjective depending on what you are using the online scanner for. E.g if you already have KAV installed, you probably want another online scanner.
http://wiki.castlecops.com/Online_antivirus_scans might be worth looking at , though it doesn't rank them.
Computer Security