Man... 'best' is a tough call. Off the top of my head I'd say 'The Unforgiven' by Metallica. Not my favourite song as such, but it's a powerful song and the lyrics resonate. If you're a singer it's also an enjoyable song to sing.

I may be bias, also. After my dad passed away, one of his friends used this song as backing for a video about my dad. It took me a long time to be able to listen to this song without crying because of the association, but I'd loved the song prior so I had to come back to it.