Just a pipe dream... 2012, possibly?
Kyl/Tancredo would be optimum, and probably do just as well as Goldwater... Any way you cut it, guys, you're not going to get an absolute conservative in the White House. Folks on the ends of the spectrum (such as ourselves, Kyl, and Tancredo) may have limited local success, but, quite simply, alienate most voters. And when that happens, we end up with a rat.
So, in conclusion, Giuliani or Jeb would be the most electable in 2008 (VP slot up in the air), and a hell of a lot more conservative than Hillary. Rice or Powell probably isn't likely.
Kyl has a lifetime ACU rating of 97. Tancredo has 97, and Toomey has 96, FWIW.