.com is king and probably always will be.
.co.uk is good for targeting the UK market. If your business is not worldwide, and you are trying to rank high in google.co.uk, I would recommend this extension.
.cc is way down on the list of viable domain extensions. I wouldn't recommend using a domain that no one ever heard of. Even if you tell them isell.cc, more people than you would like will forget the .cc by the time they get to their computer and go to isell.com
.cc might be good for Credit Card related sites, but I can't recommend it for much else.