Well, it's a really hard question, because it changes all the time. But if I have to pick up exactly one song, I think, it must be Madonna's Bad Girl. Such great memories from my childhood, when I watched this brilliant video million times a day! :-) I know, this song can be far from being perfect, but I also know that there is no song I love more than Bad Girl.
(PS. Though the first thought was about Zombie, I eventually changed my mind.)
day 02 - your least favorite song
Hm, I used to have a tag "hate this song", but about a week ago I decided to delete it, because.. just don't know :-) And about the question - my all-time most hated song is Umbrella by Rihanna. Really annoying, and the singer's vocals sound like she is sick and going to barf X_X
day 03 - a song that makes you happy
Ohh, there are many songs that make me happy, but the most working one is Lady Gaga's Bad Romance :D
And I think I must mention P!nk's Stupid Girls…