I've used all of the following: ClientExec, WHMCS, iHost, iPanel, Account Labs Plus.
ClientExec is good, and has many of the same features that others has, but the support is sloppy at times, and there is little to no effort on responses, mostly template replies.
iHost/iPanel are for the "cheaper" host, lets admit it, the basic template looks hardly anywhere near professional, and you have to make quite a few changes to make it look good.
But I have to say. WHMCS (http://www.whmcs.com
) is by far the best I have used. WHMCS already looks good, and with a few quick and simple changes to one template, you can integrate it with your site.