I recommend Gentoo, but I wouldn't USE Gentoo.
See, I don't like Linux. From the memory management to the lack of standardization, to the plethora of mutually incompatible package managers, Linux is too much of a mess for me to use, let alone like.
I recommend Gentoo because it provides Portage. Portage is directly inspired by the BSD ports system. For those of you not familiar, I'll break it down: On any *nix box you can install software from source code (IE, build it yourself) or from binary packages (like installing a Windows program). On a Linux box, these two things are generally (though not always) two completely seperate things.
On a BSD box, these two systems are almost identical. If you want to install a package (lets say, php4). As root, you just type:
# pkg_add -r php4
Your machine goes out and fetches the package and installs it. You can do pretty much the same thing with Linux style RPMs or deb packages, too.
If you want to build from source, you just do this as root:
# cd /usr/ports/lang/php4
# make install && make clean
That will fetch the source, compile it, and install the software. Not much different than using a package, really.
With Linux, when you download a package, you get whatever the package maintainer thought was a good idea to put together. When you download a package on a BSD box, you get exactly the same thing as you would have gotten from installing from the source in /usr/ports. This means that you can depend on identical behavior from both sides of the software management system. This is a very good thing from both a systems administration point of view AND a user point of view.