I personally think HelpScout is overhyped, especially it only offers a KB feature through a (paid) plug-in, but I haven't tried it so it's just my impression (they certainly do good marketing though!)
I use Freshdesk Blossom plan ($19/m/agent) which is cheaper than the Zendesk Regular plan (the first one to provide domain mapping, such as http://support.mycompany.com )
I haven't had too much problem getting used to Freshdesk and they've made some good UI improvements overtime (such as sending a response AND setting the ticket status at the same time - probably something Zendesk does too). I also keep the impression that I struggled a bit with ZD when I used their trial a while ago.
We also use Grasshopper, but I don't know if/how it works for a call center (I guess they probably have an offer for that too).