From time to time I use Eset online scanner,and have had no problems. A lot of the others fail to start,and/or complete. So,my vote,and preference,is for ESET.
I've used http://anubis.iseclab.org/?action=home and http://www.sandboxie.com/phpbb/viewtopic.php?t=6557 Never used an online scanner for an entire system...
Tried them, but they don't seem to perform as well as local scanners. Therefore None, excluding cloud programs, and multi-scanning services for single files.
Well I do not use any Online Scanner . I just have the Anti virus named " Avast" for security purpose . It also scans my computer for virus detection .
Kaspersky online scan once caught an additional malware on my sytem that clamwin, microsoft sec essentials, and malwarebytes missed. ...tried to find it the other day, but couldn't.