I've tried various scripts but WHMCS comes out on top for me. It's a mix of good value and reliability, they've been in business for a long time and proven themseleves (not without their down moments of course).
You missed Blesta We use that and haven't had any issues and it's 99.6% open code, so we can make changes we need. I used to be a WHMCS customer until May 2013, just before localhost.re showed how bad their code was. Whilst I was lucky, a few mates wasn't and had to put their systems offline. But cPanel own 49% of WHMCS so should kick them into place but they would need a full rewrite to do so, with an audit. But people use what they like, Security or Features. Myself I like security.
Hands down, WHMCS. I love the extendability of it. I love the fact that it utilizes smarty. It's not the most 'secure' system in the world (we all know there have been hacks), but if you know what you're doing, you can easily make it more secure.