I'd have said WHMCS without doubt a few months ago, however now ClientExec really has changed (for the better!) The new update (I believe 4.1) has a new front end which looks quite nice and the back-end was completely redesigned recently and so it's much nicer than it used to be.
I think the support system could do with a bit of work, but apart from that, for what we want to sell, ClientExec is very good!
- Europe based shared hosting solutions
Fast & friendly UK based support, with 99.9% uptime!