For me, it would be definitely Beethoven's 9th Symphony.
There hasn't been a greater musical achievement of such magnitude ever since IMO.
Obviously this whole thread is of extremely personal opinion, but, I have to ask, isn't there the danger of confusing quantity with quality?
I kinda suspect the same thing happening with the poster who suggested Whirlwind, which is also over an hour of audio. Are those two pieces of constant high quality, or is it just easy to blend out the weak parts and remember the strong ones? If the piece is long enough, there's bound to be enough sections you will like.
When I suggested for example A Day in the Life or Hotel California, I thought those two songs shine because there are no bad parts. They aren't crazy long, but in that time frame they deliver salvo after salvo of awesomeness.
I used to make the same argument about A Change of Seasons / Six Degrees and Blind Faith / Lines in the Sand years ago.
I'm quite the sucker for buying into the colossal reputation that certain albums have. Most, of course, just become good/great albums that don't have any huge lasting impact. A few painful cases have crashed and burned under the weight of my anticipation and spectacularly failed to deliver on their mythos - the two biggest ones were The Wall and Sgt Peppers.
I never had greater expectations for a piece of music than Beethoven's 9th. I kid you not, when I bought it, I didn't dare listen to it for nine months. It just sat there waiting amongst my other music until I could muster up the balls to put it on.
And it didn't disappoint me at all. Looking back, I guess I should find that incredible, but it's just that good, from start to finish, which isn't something I'd say about many of Beethoven's works. In fact, the Moonlight Sonata has an amazing first movement, a brief bridge of a second, and then a jarring third that, to me, actually detracts from the impact of the first.