My top track is Bloc Party - Banquet, but that's only because it was my top track on iTunes by about 50 plays (now by 80 plays from 2nd place, with 536 plays), and I felt that my last.fm should mirror my music tastes slightly more than it did, so I left it on repeat for a few hours.
But, I listen to most of my music in albums, so generally, especially if you look at my weekly top tracks, its just the song I start listening to the album with that gets the most plays.
Tracks 3-7 in my top most played are all from the same album, and theres more from that album at 22, 28, 29, 32, 34, 37, 47. And that's the entire album. And I only love the few tracks that are 3-7, but I've listened to the album a lot, so the others get played too.