What attracts you to a song? Is it the melody? the lyrics? the singer?
Those questions just popped into my head a moment ago.
For me, there are a few artists whom have unique voices where I could listen to them singing anything - like Lisa Brokop, Patsy Cline or Johnny Cash.
Most of the time, I think I'm drawn in by lyrics to songs. Maybe that's why my favorite genres of music are the old country, folk, roots, Americana, and a bit of rock. Those genres are mostly inhabited by the singer/songwriter - those special people who can turn words into the greatest stories with just simple melodies.
Along with great stories and simple melodies comes the final ingredient to the recipe for a good song. That would be harmony. Even a simple two-part harmony can change a plain song into something wonderful.
There are a handful of great singers who do harmony exquisitely, such as Sheryl Crow, Emmylou Harris, Vince Gill, Patty Loveless, or Trisha Yearwood. Or how about those singing duos/groups, like The Judds, Ann & Nancy Wilson, or Amy & Emily of the Indigo Girls. Singing harmony isn't easy. It's a gift that few can master.
So, that's my answer to the question. Either give me something from a unique voice, or put together the recipe of a great story, simple melody and a bit of harmony. You'll have me hooked from the first line.