Sorry - I probably sounded a little arrogant and dismissive too. I blame my job.
Seriously, though, there's a certain amount of truth to it, and I think a "decision tree" is a better answer than "try this song." My experience as both a student and, occasionally, a teacher, is people will practice the shit out of something that interests them, and will not practice something that doesn't.
So, I think there are two factors to consider... The studen't interests (and abilities, of course), and the particular technique or concept they're trying to learn. With that in mind, PirateMetalTroy's possibly slightly flippant answer really isn't such a bad one - say you've got a student interested in heavier, shreddy stuff, and you and he decide you want to work on his string skipping. "Erotomania" would be an obvious suggestion, as would a fair amount of Paul Gilbert material. Or, a intermediate level guy interested in sweep picking - there isn't much "easy" sweeping material so my first inclination would be to show him a couple shapes, let him learn those, and then write a short little piece stringing a bunch of them together for him to practice (I've written myself a couple) before moving onto something arpeggio-based like Becker's "Serrana" which isn't bad if you take it slow.
So, a slightly less snotty, more fleshed out answer - a song in a style that a student would enjoy that demonstrates a technique or concept you're trying to teach him, with the song in question changing based on the inputs.
...and I've always HATED House of the Rising Sun. :lol: