"When the Levee Breaks" (Led Zeppelin)

So here's the thing: I think "Stairway to Heaven" is overrated and overplayed.  It's a decent song, but it's also self-consciously grandiose in a way that suggests the band was trying to prove something.  And then there's all the brouhaha over whether or not Jimmy Page stole the opening guitar line/chord progression from the 1968 track "Taurus" by the band Spirit—a band that Led Zeppelin opened for on its 1969 US tour.  (Listen for yourself, and you make the call.)
The real epic on Zep's 1971 untitled fourth album is "When the Levee Breaks," a dramatic reworking of the 1929 Delta blues tune by "Kansas Joe" McCoy and his wife, "Memphis Minnie" Lawler, about the Great Mississippi Flood of 1927.
In fact, the only thing the band held onto from the original was the lyric.  (In this particular case, they had the good sense and decorum to acknowledge "Memphis Minnie" in the writing credits.)  Lawler's words paint a harrowing picture of the 1927 disaster: rising floodwaters and breached levees forcing throngs of people from their homes while others downriver awaited their fate and worried.
The lyrics are eerie enough on the original recording; but in Robert Plant's hands, they have a gravity that's downright apocalyptic and an electricity that's irresistible.
Granted, it doesn't hurt that he has John Bonham behind him, playing one of the most thunderous, resonating beats in all of rock (an effect that was achieved by Bonham playing his drum kit at the bottom of a stairwell and engineer Andy Johns recording him from two floors above).
More than anything, though, I love this song for the little melodic shifts and genius production touches (like the backward harmonica line) that help foster the sense of building tension.  Especially leading up to the final minute, when Page's guitar lines surface, pan across, and then get submerged—not unlike watching debris in floodwaters.