Well, I think that it just depends on what you want. Basicly every
distro offers same 64bit support because it's the same kernel and the
same software you are running. It's all about the taste and testing
phases. I have found Debian Gnu/Linux(also Ubuntu as they use the same
codebase) amd64bit binaries very stable and working. Gentoo Linux has
also a very good binaries as you compile them by yourself.
Debian is a good shot as it has fine precompiled packages. Fedora4 has
also a 64bit support, but since Fedora3 had unstable 64bit binaries and
I have read the Fedora4 is a bad anyway with lots of bugs.
Slackware 64bit version might be a good one too. But I think that the
best 64bit distro at the moment is just basic Debian Gnu/Linux. As it is
something between the Gentoo Sourcepower and Redhat Usability.
-Antti-