"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.
Sending and Receiving Faxes on a Mac
If you need to receive a fax on your Mac, there are several services which all work fairly equally in quality of service. There are a few applications which allow you to use the traditional method of connecting a telephone line to a modem inside your Mac, but at this point I don't recommend those solutions because they only work if you have a phone line available. If you use an online service, you can send and receive a fax anywhere you can get an Internet connection.
Here are a few of the best fax services for sending a fax on a Mac.
MetroFax gives you a dedicated fax phone number, so you can have people who use traditional fax machines send you messages. For $2.00 per month, this can be a toll free number. You can opt to have faxes sent to you either as PDF or TIFF attachments. Either can be opened with Preview on your Mac. For $12.95 a month, you can send and receive up to a combined 1000 pages. After hitting the 1000 mark, each additional page is $0.03. MetroFax offers a solid combination of price and value compared to other offerings listed here.
MyFax may be your best option if you only occasionally receive faxes. They also allow you to send and receive faxes via email, but pricing is more expensive as your volume increases. If you need to receive 200 pages or less per month, or send under 100, the $10/month fee is your best value. To send an additional 100 pages the price jumps another $10/month. MyFax currently lists 42 countries falling within their "free fax zone", which means you can fax to any of those countries as part of your monthly fee.
RapidFax has many similarities to MyFax. Pricing is $9.95/month for 300 combined send and receive pages, which matches up with the 200 received and 100 sent of MyFax. Additional pages are $0.08 each. Like other services listed here, you get a dedicated phone number which allows people to send you faxes from traditional fax machines. If you need a toll-free fax number, RapidFax allows you to have one for no additional charge, although unlike MetroFax, you can't get both a local and toll-free number for the same account.
eFax was my primary fax solution for several years. I have since switched to using MyFax. If you are more comfortable with using separate software for faxing from your Mac, eFax might be your best bet. In addition to supporting fax via email, eFax also includes a software app you can install on your Mac. The convenience of a fax app will cost you, because eFax is the most expensive service of the bunch. For a $16.95 monthly fee you can receive 130 pages per and send 30, with received pages adding an extra $0.15 each over 130 and sent pages tacking on an additional $0.10. If you pay for a full year, the price drops to $14.13 per month, but eFax remains the most expesive solution in the field of choices.
Choosing a Fax Service
In reviewing the choices above, if you receive many fax pages monthly, MetroFax is your best price per page option. For the occasional fax, MyFax or RapidFax are better suited from a price standpoint. For desktop software, eFaxis the only solution for your Mac. If you just need to send a fax once or twice, use FaxZero.com.