Going to California is my favorite song on Untitled, though I love When The Levee Breaks. Of course, as I've stated a thousand times, my favorite Zep song is Kashmir. For those of you who have yet to taste the fruits of Zep beyond Untitled, then I recommend a little visit to House of the Holy and PHYSICAL GRAFFITI (O YEA!)
Highlights of Houses of the Holy: The Song Remains The Same, The Rain Song, Over The Hills And Far Away, DANCING DAYS

, and that reggae song D'yer M'ker or something.
All of Physical Graffiti is wonderful--even the filler, I'd venture to say.
For songs from other albums, I'd go with Achilles Last Stand and ALL OF MY LOVE (best lyrics they ever wrote, I think).