OK, on to my pick. The top three picks are highly worthy of the greatest human achievement. So, I think, is this one:
Symphony No. 5 in C minor, Op. 67 by Ludwig van Beethoven (1804-1808). Certainly the most famous musical piece ever written.
A couple of notes:
1. I had always thought this was about fate or death, but the Wikipedia article states that there is no evidence of this; that it's based on an after the fact essay by a man named Schindler (not the humanitarian.)
2. I didn't realize how long it took him to write it. (5 years.)
3. The symphony is scored for piccolo (fourth movement only), 2 flutes, 2 oboes, 2 clarinets in B flat and C, 2 bassoons, contrabassoon (fourth movement only), 2 horns in E flat and C, 2 trumpets, 3 trombones (alto, tenor, and bass, fourth movement only), timpani (in G-C) and strings.
(This is also news to me, though probably not to everyone here.)