I voted for Pink Floyed under the assumption that you meant Pink Floyd as the best of those 4. As for the rest, Zep just insn't all that, Iron Maiden was awesome when I was 13 (ie: 15 years ago) and Metallica stopped being cool with the release of the Sellout album, errr I mean Black album.
I have to agree, Iron Maiden and Metallica shouldn't be compared to Led Zepplin and Pink Floyd, the last two are in a league of their own and were very influental on the music industry where the first two were just normal bands, had their run and went.
Should have been the Beatles, maybe the Rolling Stones, groups like them that had an impact. Even Uriah Heep who have been going for over 30 years and released over 30 albums and still going.