That would be one huge poll!
It's a really tricky question I have to say, I already had to think long and wide about it on Facebook.
The song that is most representative is The Great Escape in my opinion, but mainly because it's so long and so there was enough time to put all that is so amazing about the band into it.
But to introduced the band to someone it would be plain wrong.
It also depends on whom you want to introduce to the band. Yesterday I introduced a friend from Brazil to Seventh Wonder. And I started with the ballads, because I knew she would like that, so I went in that order:
1. Pieces
2. The Long Way Home
3. Tears For A Father
4. King Of Whitewater
5. Alley Cat
6. Not An Angel
and so on...
I think I convinced her, she hinted she wants to start with buying Mercy Falls first.
I doubt I would go the same direction with other people.
But I noticed I had a problem with picking songs from Mercy Falls because I think you should listen to the album as a whole to really appreciate the beauty.
So I think if I wanted to introduce someone to the band I would go with Taint The Sky, The Black Parade and Alley Cat. It's a nice summary of the last three albums.