Definitely WHMCS. While it does have a lot of features, the admin area is very easy to navigate. It's also highly customizable. I have it embedded in my site design and you can barely even notice that it's billing software.

As far as ClientExec goes, it is definitely simple and can get the job done. However a big draw back for me (not sure if this is still the case) is that you cannot credit a customers account. You basically have to manually create invoices with no charges or something along those lines.