If this thread had been 'favourite track' it would have been as utterly ridiculous as asking a bibliophile to choose their favourite book.
But since it's 'most powerful track', still in the teeth of stiff competition I'll nominate the cover version of Jimi Hendrix's
Little Wing by Derek and the Dominos, a short-lived (1970) side project of a whip-thin, frail, fragile, increasingly physically and mentally unwell and early-stage heroin-addicted 25-year-old Eric Patrick Clapton at the absolute top of his form. I'm somewhat of a traditionalist who holds it as axiomatic that cover versions are very, very rarely better than the originals. And it's a rare and brave (or foolish) musician who takes on Hendrix. But I think that this is a rare occasion where Hendrix's dippy trippy hippy ballad, for all its beautiful chord structure:
http://tinyurl.com/y894ujj6is turned - with Clapton vocally drowned out by the magnifcent Bobby Whitlock, and Jim Gordon walloping seven shades out of his drumkit - into a sonic advance on Stalingrad never, to my mind, equalled.
This is what absolutely desperate twenty-somethings living on - in the recording studio in Miami - bags of cocaine and bottles of Johnny Walker on top of the amplifiers and not much else can do:
http://tinyurl.com/y76vlnep