Like Smurfy, I find it impossible to pin down a favourite song because music, to me, has so many functions. For a song to really grab me, it has to be good lyrically and orchestrally (??) and also have a political or cultural dimension. I have mentioned few of my favourites before but here are a few more:
For Your Love - Yardbirds
Changes - David Bowie
Loving You - Minnie Ripperton
She - HAS to be Elvis Costello version though
More later.