I'm just a bit too young to know what was going on in 1971 from first hand. My musical awakening took place in the cauldron of post-prog punk, and while I still appreciate some of what was going on in the late 1970s I discovered prog rock when it was deeply unfashionable. As far as 1971 is concerned; the standout for me is Echoes, actually the whole of the Meddle album, with Aqualung and Heart of the Sunrise by Yes as close followers. Another great album, although not prog by most standards, is Deep Purple's Fireball - particularly tracks being Fools and No No No.