Sorry, but Led Zeppelin II is by far the most influential album and the early model for heavy metal. It spent 7 consecutive weeks at #1 in the U.S. and is Certified 12X Platinum. Plus it's my favorite, followed closely by IV.
All solid points, but the first album was the first one of its kind. The second built on that and achieved greater popularity, but "I" was the root of that popularity.
That being said, IMO they are both ahead of Houses of the Holy--a great album in its own right--so I will continue to vote for "I" a bit longer...
Led Zeppelin 
Led Zeppelin II 
Led Zeppelin IV 
Houses of the Holy