I love 'Imagine' by John Lennon.
The Doors 'People Are Strange' may be a good song as well since it deals with the strangeness of us humans and that feeling of aloneness we all feel at times.
I wrote a song a few years ago called 'Everyone Is God' which basically says that whatever god is, we are it. If god is nothing more then the laws of nature in motion - so be it. In the song I say that god is "life, death, and everything in between." Now, imagine that.