For me it has to be 'White Man in Hammersmith Palais' by the Clash. It completely blew me away when I first heard it and even now I only have to hear the opening bars and I'm back to being a snotty fourteen year old with a safety pin through the lapel of his school blazer...

And yeah, of course they were a folk band!

As for the best ever of all time blah blah either no such record exists or a hundred thousand variants exist. see, it all depends if we're talking about your best or mine!

Cheers

Nigel