Very hard!!!

It would have to be Ritual from Tales from Topographic Oceans by Yes.

I heard them do this in 1973/4 and it blew me away completely. I've been in love with it ever since. The build up that ends in the "At all, At all" section is breathtaking and when they performed it on the Magnification tour recently I thought I'd died and gone to heaven.

Bless you Jon Anderson!!!!