Avoid BoxBilling at all costs. I used it for a spell as all I wanted to do was generate invoices and keep track of accounts for a non-hosting related matter and I wanted to walk blindfolded into uncomming traffic. Okay, that may be an exaggeration... but only a slight one.
HostBill I actually quite like. I think the user interface is quite snazzy which is important, but the admin interface was a bit of a mess in my opinion. It had some real great intergrated features and a better ticketing system than WHMCS, but the developers are or were bi-polar when it came to pricing. When I used it I was renting a license for like... $8/mo or something. They have changed the pricing so many times and it's skyrocketed so much. I don't think an expensive helpdesk is an issue, and in fact, I think the cost of entry to the market should be higher in some regards (assuming the tradeoff for higher priced products is better products) but it just wasn't quite all there yet.
WHMCS is still the go-to standard. It works. I hate the ticketing system though and the inability to do multi-tasking functions like you can in Kayako. (EX: Respond to ticket, unflag it, move to new department all in one action and page load). If WHMCS had a better ticketing system I think it'd be pretty decent. For basic invoicing and customer management, it's alright.
Blesta: Never used it. It looks alright. I don't think I've ever even seen it in the wild. Any one using it? I'd like to go through the ordering process on your site and see how it is for the end-user.
ClientExec: Been too many years. I honestly don't recall much detail about using it.
I'm not really sure of any contenders. Every few months someone comes along and says they're working on the next best thing in web-hosting billing, client management, accounting and ticketing and then not much comes after that.