I'm probably a little biased but love WHMCS. We use it and love all the features of it and how it just works.

Have had a chance to try some of the others like modernbill and this just feels easiest to use as well as powerful to handle everything we need for it to do.

A lot of those listed are good billing software, I think at the end of the day it really boils down to what you just feel most comfortable using.