I think you've got most of the best covered, although I would add "Good Times, Bad Times", and the riff after the slow blues type intro for "Ramble On". But my number 1 Zep riff would have to be "The Wanton Song". The riff is killer, but so is the quick break, where the band stops, and Jimmy does a "fade in", on a power chord, just using his volumn knob, and maybe even some kind of volumn pedal? The tone and power of that fade in is simply amazing, even now.
On a side note, can it get much better than the double whammy of "Heartbreaker/Living Loving Maid", back to back with no break between? And even though its a slower riff/song, how about the great guitar and vocal melody of "Ten Years Gone"?
Man, just talking about this makes me want to go listen to a bunch of Zep!!
Ive always been partial to 'In the Evening' and 'Over the Hills and Far Away', although the latter being acoustic one may argue of the riffness therin. :-)