That depends on how much work you're willing to do to make it fast. Just out of the box, I've been told Slackware and Gentoo are the fastest. If you do a base (text install) of Debian and then build your system on top of that, it is very responsive as well.
You'll also gain some speed by recompiling your kernel and cutting out anything you're not using.
Speed on the desktop will also depend on the desktop environment or window manager you choose. If you want it to be fast, don't use KDE or Gnome, use something like IceWM, windowmaker, or fluxbox.
And the thing that makes linux really fast is more RAM.