Damn that's a good album.
I'm going to say "Stairway to Heaven" is one of them, there is a reason why that is considered such a special song.
`Black Dog is another one, that is such iconic Led Zeppelin, it rocks so hard, and it's so original, can you think of any other songs that are like Black Dog?
Going to California is really beautiful, So that's another pick for me.
And my final pick is When the Levee Breaks, because it always reminds me of good times with my band.
I was a guitarist, and during breaks in rehearsals, some of us would try getting on the drumkit and attempt to play it, for fun. I once had a bet with our drummer that I could learn to play the opening drum part of When the Levee Breaks within 24 hours. I almost won it, but not quite, I flaked and couldn't play it cleanly. But the opening drums of that song are etched in my brain.