Okay, I'm a music teacher, and I'll admit I have very strong prejudices after hearing some music performed hundreds of times (most of those times not very well).
  For classical?  "Fur Elise" (you've all heard it even if you don't recognize the name).  For Irish?  It's a tie!   "Danny Boy" and "Too-ra-loo-ra-loora".  For Christmas?  "Here Comes Santa Claus".  From Broadway?  "Tomorrow" from Annie makes me gag.
  Alas, there are more.