Tom's root boot
http://www.toms.net/rb/
Floppy distro that works great! I've depended on it for lots of things, even when I've had a full feature distro available to me.
As far as running something with 64MB of RAM, don't count on running X with it unless you're running a relatively simple window manager (FVWM2, TWM, or possibly Enlightenment v16). GNOME and KDE are memory hogs.
That said, I'd guess that the most versatile full feature distro would be Debian. Of the major distros, it tends to be behind on advanced functionality by a year or two, but it's stable.
I know you have an old machine, but you might try to get extra memory for it. If you could bump it up to 128M or 256M (your machine might not be able to physically handle 256M), Linux would work very nicely.
Good luck!