Although there is no official Unix / FOSS package manager for Mac OS X, it should be noted that MacPorts was created and is still primarily maintained by Apple's Mac OS X / Darwin engineers. The macports.org website is hosted on Apple's servers, as part of Apple's "MacOSForge.com" project. Because Mac OS X / Darwin is *BSD based, many of Apple's Unix-layer software engineers come from the *BSD traditions, so they modeled MacPorts after the *BSD "ports" model.
Fink was the original open source package manager for Mac OS X, but it does not have the close organizational ties to Apple, and it chose to follow the Debian Linux apt-get tradition.
I'm not sure any of the details above is enough to recommend one over the other. I would say it comes down to which model you're already more familiar with. If you're more familiar with apt-get, go with Fink. If you're more familiar with BSD "ports", go with MacPorts.
As others have said, you're best off leaving Mac OS X's default Unix tools alone. Fink and MacPorts have learned that lesson already and install their own copies of things in their own directory hierarchies (/sw for Fink, /opt for MacPorts). Homebrew has not yet fully learned that lesson, but they will, as soon as some user uses Homebrew to update the built-in copy of, say, Perl or Python, and it has some unforeseen compatibility problem that breaks built-in pieces of Mac OS X, such as installer packages (Mac OS X .pkg bundle files) that use Perl scripts to perform some of the installation tasks.