"Time", by Pink Floyd. I have been giving "Dark Side Of The Moon" a good old thrashing lately. For many years I had something of a love/hate relationship with it: as an aspiring young antisocial post-punk nihilist (in my own mind) I was against it and everything it stood for. The problem was that by then its spell had well and truly been cast: I had been listening to it for a long time and had fallen in love with it. So I avoided the issue by just not playing it.
I have always been obsessed with the moment in this song, around the 4.30 mark (4.05 in the live clip I have linked to), where the guitar solo changes; it remains a guitar solo, but it spins on the head of a pin and becomes something else entirely. Without blinking an eye or missing a beat. I still don't know how they did that.