I'm using SwiftModders WHMCS Admin Theme, and it is simply beautiful. This is my default backend admin theme for my service and it has been for some time now.
I must had used about 10 different backend themes and they were all pretty much a no-no. I don't want to offend other theme makers but my taste for well designed backend themes is pretty high.
I'm using this interface on a day to day basis, it needs to look good and be practical, otherwise it's going to make my work painful.
I can say without a doubt these guys knows what they are doing and they have fair prices. If your business depends on using WHMCS all day, you absolutely need to change that default theme, it's hideous and horrible. SwiftModders WHMCS Admin Theme changes that completely. I wouldn't like the system the same way if it weren't for this.