Hands down, WHMCS. Why? A few reasons
#1: Smarty templating . I can't get enough of smarty, I love it, have loved it for years.
#2: Extendability . I love that if I need to add something to WHMCS, I can (and have) added hooks, functionality, you name it. It's incredibly easy if you know what you're doing.
#3: Ease of use. Some would say it's a bit complicated, but I don't find it so much. Of course, I'm a tech, by nature, so I work through things a bit different than most