Some songs are great because of their melodies, some are great because of their musicianship, some for their lyrics, some for their place in rock history. There are many reasons a song can be considered great. For me, Immigrant Song is all about that propulsive beat. There's a 'primitive' quality to it, from the driving riff to Plant's opening primal scream. On the one hand it coudn't be a simpler song, but on the other, it gets right down to the basics of rock music: rhythm. In this way it reminds me of Tool's 'Triad', another song that sounds 'simple' on the surface (almost literally just a one-note riff), or Pete Townshend's brilliant 1-note guitar solo in I Can See For Miles. Sometimes it's good to be reminded of the power of just one or 2 notes, and Immigrant Song does that for me. It's not their best song, but it is one of the best of its kind.