Oh come on, everyone says Morgan Freeman. But really, for all his fantastic vocal chords the man can't improvise. Narration is fine, but it would get dry eventually and you'd get bored. Personally, give me someone who could make me enjoy their voice, as well as the sound of their voice: Billy Connoly, Robin Williams, Jack Black. One of those voices in your head (Or all three, for a cumulatively awesome panel of commentary awesomeness) and you'd be set for life.
*Picks up bacon*
Jack Black: He picks up bacon, not the apple, because he is not a ***** to science and their voodoo health magic.
You can't kill that metal.