I voted "other' - for Mystery Train.
I know it's not really a GPN song - but the cover versions I have heard are just great -
 a song that makes your body boogie/dance without you having to try.
I'll also agree that as a VERY close second comes ANY version of Cortez the Killer.
was exposed to Grace singing this song here:
 http://www.youtube.com/watch?v=paeNnR33i5Q&amp;feature=player_embedded.
the trumpet gives it a very different sound. Grace looks SO young!
also, was extremely happy to hear GPN do this song w/out any special guests in Nov 2007, 
in Flagstaff, and it wasn't the encore. I remember a certain KC helped me out by taking a 48 bit CD I sent him, and putting it into a format I could play - thank you again for that!