I'll come clean and say I don't know nearly enough of Deep Purples catalog. But maybe that is a result in and of itself? I can only submit that I think Zep comprised 4 musical genius' in one band. Each member has a calling card in a song of theirs that has, and continues, to stand the test of time. As far as I can remember ( I'm sure I'll be corrected if I'm wrong as DP fans are ... passionate

) they were a lightning in a bottle band. Kinda like GnR ( Oh boy NOW I've done it!! ) . 70-72 they were the shiz. Then they had an occasional song, but nothing that really took. Smoke on the Water and Highway Star are their calling cards, yeah? If that was all I had to my bands name I would be thrilled. I just don't think I could rank myself, musically, with Led Zep.
In the end it's subjective. I will say I think DP had a wider area of effect. Led Zep was mostly an American phenom. DP were pretty much world wide.