Thats easy, Westlife- World of our own! About 5 years ago, and its still on there to this day on my latest iPod/mp4 player, since I have both.
Why? Because well the song is amazing! Its a Westlife party song, and makes me feel alive every time I listen to it! Just can't get enough of Westlife , my music playlist is full of Westlife and nothing more! I listen to other music too but not much, I say I have a wide range of music taste but then for a song to get on my Mp4 player it has to be amazing! And music these days is nowhere near amazing!
I didn't get an iPod till quite late, I was happy with my walkman! (now you all know how old I am!)
However the reason I got an iPod was for mainly running, so I think one of the first songs I put on it was Queen 'Stone Cold Crazy. It is an amazing song to run it has such a fast beat, it really gets you going!
Though I do agree with Dan, my taste in music has changed dramatically. I now have anything from Eminem, Lady Gaga, Michael Jackson, I still have Queen and I even have some of the Muppets soundtrack (I could blame my boys for that, but I will be honest they are happy songs and I enjoy them!)