Led Zep is up there. Might not be #1, but it's in the running.
A few others that are in the running:
The Beatles - Please Please Me
The Who - My Generation
Jimi Hendrix Experience - Are You Experienced?
The Mama's and Papa's - If You Can Believe Your Eyes and Ears (set up the SoCal sound for years)
Boston (self-titled)
Ramones (self-titled)
Black Sabbath (self-titled)
R.E.M. - Murmur (arguable, but rather than making a huge immediate impact, it was more of a slow burn in changing the direction of rock, and single-handedly created the 'college music' or 'alternative') genre).
Elvis Presley (self-titled) (how can this one be left out???)
Steely Dan - Can't Buy a Thrill (thanks for the reminder nailer)
Many bands left a huge impact on rock, but it can't be said that they did it with their debut album. Case in point: Nirvana. No one would argue the legacy they left, but their '89 debut album, 'Bleach', was largely ignored until the release of '91's 'Nevermind'. Only then did people decide to mine that vein to see what else may lurking in those shadows.