Man, so many songs to choose from.
When I was in to power lifting Immigrant Song was my favorite. That opening vocal falling into the first verse would get me hopping up and down in the squat rack. Then when Plant sings, " Valhalla, I am coming!" I would come up with the weight. That verse drove me to my first 340lb squat at 190lbs.
As a teenager Ramble On was my favorite for a while. I had a messed up family life to go on top of the usual pubescent bullshit. So, to me that song was like a fantasy. "For now I smell the rain And with it pain, and it's headed my way Sometimes I grow so tired But I know I've got one thing I got to do. . ." and then my best friend moved half way across the country. So, another verse became very emotional for me.
Got no time to for spreading roots
The time has come to be gone
And to our health we drank a thousand times
It's time to ramble on
I just wanted to escape my life and that song encapsulated my ultimate dream when I was fifteen.
Moby Dick was also a big favorite of mine. I had never heard drumming like that until I got my hands on a copy of Led Zeppelin II. It was explosive, brutal, refined, and complex.
Out On The Tiles was a favorite because it didn't sound like a love song, but it was. The arrangement makes it sound like the guy might actually be a stalker, but if you don't think of it as having an unreliable narrator, it was a great change from the 80s and 90s power ballads I grew up hearing on the radio.
Oh, and Nobody's Fault But Mine. The way Plant's vocal riffing quotes that futuristic scifi guitar sound, then there is the false ending to build tension, and bam it the rhythm section falls on you like a ton of bricks. Then they keep playing with the tension and release through out the whole using variations on it to keep things uncertain and build a sense of excitement. It was definitely arranged and produced by somebody with a great vision and understanding of musical dynamics.
ETA:
This was not meant as an entry. I just wanted to keep the thread alive. lol