FicWad: fresh-picked original and fan fiction

To say the least, this is one of the most original introductory chapters I've ever read here. The mere idea of someone actually trying to have a conversation with Binns already deserves a big kudos.

Now, as for your question, I dunno how closely you want to stick with canon. If you decide to do so, I would go with Riddle; he had the greatest potential to turn into a ghost, what with his horribly distorted soul et al. However, if you want to venture into AU, you have more possibilities.

Harry IS on my list of candidates; he's known to have temper issues. However, I'd simply LOVE to see Ginny Weasley as a ghost; with the anger management courses she never attended, she would be the PERFECT one to scare the willies out of Davies on more than one occasion.
Luna has all potential to be a ghost but she doesn't fit the "temper" issue.

Either way you choose, this story goes into my favourites.