Borrowing from both Spirit's "Taurus" and The Chocolate Watchband's "And She's Lonely," it stands as the single most famous rip-off in history.
Be angry if you want, but Zeppelin has a very long history of "borrowing" riffs, lyrics, and in some cases entire songs without remembering to credit the original artists, thus denying them royalties.
I fully expect many thumbs-down from the willfully ignorant, but know that I still listen to them despite their thievery.
EDIT: You claim they've had "zero impact"? Well I think directly inspiring such a now-famous song counts as impact. Jimmy Page certainly thought they were something worth imitating (Zeppelin opened for Spirit on a few occasions back when they played Taurus, and has admitted the influence).
Don't blame me for your own ignorance. I don't care if the average man on the street could name those bands any more than I care if they could tell me who the Silver Apples are (and I'd be surprised if you could either). Nevertheless, musical influence is not measured in the number of fans you have. A common story is that the Velvet Underground's first album sold roughly 1000 or so copies altogether, but everyone who bought it went out and started a band. That is influence.