I don't think there can be a 'best song ever', but my personal favourite song is Cosmology by Rolo Tomassi
. It's just unlike anything else I've ever heard, I've spent a long, long time trawling through the internet trying to find other songs that come to close to it for me, but nothing does. I like that really spacey sound, fused with changing time signatures, polyrhythms and the most epic outro to a song I've ever heard.
It's ironic, as I think Rolo Tomassi are one of the worst bands I've ever heard, making some of the most horrible noise I've ever heard, and yet this one song they make is my favourite song ever. I bet not many people can say that their favourite song is made by one of their most hated bands!