My preference is GNU/Linux for three reasons. 1) I like the functionality in the GNU versions of the standard UNIX utilities. 2) The variety of distributions from which to chose. I really like Gentoo because of the level at which you can customize your system. 3) It's free software and promotes copyleft. I regularly use Solaris because that's the system on which I do most of my job. ZFS does a good job of handling very large files systems and files. I also use Cygwin because it gives me UNIX on my XP machine at work. It's great for those who can't live without their UNIX utilities. I also use AIX at work but it seems to lack a lot of the utilities and options that are commonly found on Linux. This may just be a problem with the default installation. I've used FreeBSD in the past and it works great. I really like Ports package management.