For the best reliability and easiest setup you should use Fedora Linux (Redhat's Free OS) and use VsFTPD as your FTP daemon.
http://fedoraproject.org/wiki/. Feodra has come a really really really long way from what it was when I last used it back as redhat 9. The screenshot of the GUI looks awesome on their website and the last time I installed fedora core 2, it was really easy. The installation is much better than windows because it doesn't treat you like you are stupid and gives you lots of options during install unlike (vista/xp). You can choose to install the graphical desktop or not to install the graphical desktop in the installer.
If you need more reliability and don't want to deal with the sometimes annoying redhat/fedora RPM packaging system, don't want a gui desktop (out-of-the-box) and would like to compile your own binaries you can grab a copy of FreeBSD 6.2
www.freebsd.org and be able to choose a whole bunch of FTP daemons to fit your liking, of course VsFTPD, WUFTPD, and proFTPD are available options in the ports collection that compile right out of the box and are rock solid. proFTPD even has a MySQL backend option that you could use for large FTP sites.
We use FreeBSD 6.2 running WUFTPD. No downtime whatsoever. No GUI overhead. No Mac crap. No Windows crap. No graphical user interface crap. Perfect server environment.
IMHO, Mac OS X has it's place in the desktop environment but will NEVER be an option on any of my servers. Espcially since it doesn't like to compile PERL DBI driver DBD::mysql, it will never be considered a server of any kind in my book. Server incompatible with "standard/mature" PERL CPAN libraries = TRASH!