"What's the best fax application for Mac OS X?"
If you need a fax application to send the occasional fax with your Mac, I wouldn't use an application at all. For the very occasional use, I recommend free fax site FaxZero. It allows you to upload a file to fax and choose a recipient for your fax without spending any money. If you need to receive faxes or want to send many faxes every month, there are some other suggestions I would make instead.