My view has changed over time. My answer now is the debut - and everytime I listen to it nowadays I am reminded of the raw power (and frustration) that I experienced when I first heard it.
Frustration? I was a very regular gig goer from 1967 onwards, and when I went to Manchester University in 1968 would go to a minimum of one gig every week. I had the opportunity of going to a gig by the "New Yardbirds". It turned out to be one of the first Led Zepp gigs and they played these tracks. One of my biggest regrets of all time!