"Immigrant Song" (Led Zeppelin)

That aaah-aaah-aaaaaaaah-aaah! banshee wail.  It's as jarring as it is invigorating.  
Case in point: I was sitting at a New York Rangers hockey game not too long ago.  It was second period, score still 0-0, and the Rangers were struggling to get momentum going.  To be blunt, the crowd was damn bored.  But the moment "Immigrant Song" and Robert Plant's vocal came blasting through the Madison Square Garden speakers during a T.V. timeout, you could feel the crowd perk up.  Not two seconds after puck drop, the crowd began yelling Let's go, Rangers! like their lives depended on it, and the guys on the ice finally woke up to earn their millions.
I bring this up because "Immigrant Song" was made for arenas.  In fact, it opened pretty much every one of Led Zeppelin's shows from 1970-1972.
As Plant notes in this video, the song was inspired by a gig the band played in Reykjavik in 1970.  Zep had been invited to Iceland as British cultural ambassadors and were set to play a concert arranged by the government.  Unfortunately, the band's arrival coincided with a massive strike by the nation's civil servants, so the concert was going to be scrapped.  But one of the universities in Reykjavik (Plant doesn't say which one) stepped in to save the day, calling off classes for two days so that faculty and students could prepare the city's Laugardalsholl Hall, an indoor sports venue, so that the concert could go on.  The warm welcome from the nation and outpouring of affection from these college students really struck the band, Plant in particular.
As he says in the clip, "We got off a plane, and we were given Iceland on a plate."
To capture the feeling of the Icelandic visit, Plant borrowed imagery from the small nation's Viking heritage, imagining Zep as modern day Nordic warriors coming to conquer new lands.
That said, the song is not the cock-rock manifesto for conquering the world that most people think it is; it's meant to be funny.  It's Plant and his pals making a joke about their public image as leaden metalheads.  Actually, quite a bit of Led Zeppelin III is a message to fans and the music press to "lighten up."  (Check out the tracks "Out on the Tiles," "Gallows Pole," and "Hats Off to (Roy) Harper" for further evidence.)

"Gone Gone Gone (Done Moved On)" (Robert Plant & Alison Krauss)

I didn't quite know what to make of Robert Plant and Alison Krauss's collaboration when I first read about it back in 2007.  Don't get me wrong; I was a fan of Plant and Krauss, individually.  But together?  (I imagined a combination along the lines of cappuccino-flavored potato chips—a crime against nature if there ever was one.)

Thankfully, I was wrong about my assumptions.  In fact, I still marvel at how naturally and effortlessly their voices mesh on Raising Sand, an album featuring beguiling covers of Americana, blues, and roots rock tunes.

My favorite cut from the album is "Gone Gone Gone (Done Moved On)," a true-to-the-original cover of a 1964 single by the Everly Brothers that captures all of the nuance of Phil and Don's close harmonies.  Krauss's voice is as pure and smooth as clover honey against Plant's bluesy, whiskey tenor.

The reason the blend works so well, though, is T. Bone Burnett's tasteful production and arrangement of the song.  It retains the same rumba/rock drum groove of the original and adds some tasty tremolo guitar, amplifying the rockabilly quotient a bit.  Quite simply, Burnett creates the perfect backdrop: it rocks and rumbles like the engine of a rod hod yet doesn't outcompete the raw horsepower of the duo's vocals.

"The Ocean" (Led Zeppelin)

I feel sorry for anyone who still thinks Led Zeppelin was a heavy metal band.  Classic Rock radio plays "Whole Lotta Love" 100 times a day, and people equate that with the band's entire output.  Even though that's like hearing only "Down By The Seaside" from Physical Graffiti and labeling them a country-rock outfit based on that one song.  
Sure, the boys could do blooze and decibels better than almost anyone out there in the 1970s.  But, heavy metal?  Hardly.
If anything, they had a strong affection for early, fun rock and roll--Little Richard, The Monotones, Jerry Lee Lewis, et al.  (I'll have to look this up, but I think maybe they even did a song about the subject.)  I think that influence is pretty apparent on Houses of the Holy's "The Ocean"--a tribute to their fans in which lyricist/lead vocalist/golden god Robert Plant likens the roaring sea of people at their concerts to an ocean.  
The whole song is done with a wink and smile.  There's John Bonham's barely audible chant at the beginning ("We done 4 [takes] already, but now we're steady, and then they went 1, 2, 3, 4...") before he and John Paul Jones trick you into thinking the song's in 4/4 before shifting to 7/8, just to fuck up your footwork.  There's Jimmy Page's little oooooh's on guitar that sound like a reenactment of a nubile groupie getting frisky with a mud shark.  And then there's Plant's a cappella lala-ing through the bridge, like he's about to teach us the letter of the day with Grover and Cookie Monster, just before everything abruptly shifts to a 50s-style doo-wop groove.
It's weird, wacky stuff.  And no one else could pull it off as gloriously and seamlessly as this band.

"Kashmir" (Led Zeppelin)

Since the previous two posts were about classical pieces, it only seems appropriate to talk about a rock track that incorporates elements of classical music: "Kashmir" by Led Zeppelin.
In music journalist and filmmaker Cameron Crowe's liner notes for The Complete Studio Recordings boxed set (1993), he states that guitarist Jimmy Page had been working on portions of the song for almost three years before the track was released on 1975's Physical Graffiti.  It essentially was a warmup piece that gradually evolved into a full-blown composition with layers of guitars, mellotron, as well as actual strings and brass, arranged by the band's bassist/keyboardist/resident Swiss Army knife, John Paul Jones.
Crowe notes that it's also one of Robert Plant's proudest lyrical moments, inspired by the vocalist's travels along a long desert road in Southern Morocco.  
Quoting Crowe's interview with Plant: "It was a single track road which cut neatly through the desert.  Two miles to the East and West were ridges of sandrock.  It basically looked like you were driving down a channel, this dilapidated road, and there was seemingly no end to it. 'Oh, let the sun beat down upon my face, stars to fill my dreams...' It's one of my favorites...It was so positive, lyrically."
In the interview, Plant also praises drummer John Bonham's contributions to the track, citing Bonham's "thrift" as the key factor that makes the song work.  
And I completely agree.  So many of Bonham's contemporaries from the late 60s and 70s were "busy" drummers; I'll put Keith Moon in that category (although, his wild man style was perfect for The Who), as well as Ginger Baker (curmudgeonly egomaniac who still has no sense of groove.)  It's Bonham's straightforward, driving groove and tasteful use of fills and punctuating cymbals along the way that give the song its sense of journey.  There's no showboating; there's only a deep respect for the song itself that comes through in every lick.
Although it is hard to pick a favorite track from the Zep canon, "Kashmir" is definitely up there for me.  Let's call it my favorite epic track of theirs.  (I far prefer it to "Stairway to Heaven," which--don't get me wrong--is a finely wrought piece of music.  It also has been played to death by FM radio and co-opted as a stoner anthem because of its veiled drug references wrapped in Olde English mysticism.  It was cool in high school, but now it can be a little too much tie-dye and Tolkien to digest.)  "Kashmir" is unique in that it perfectly combines a sense of mystic beauty and light with Page and Bonham's signature thundering crunch.  It embodies the very name of the band: Led Zeppelin.