I guess if you have to choose an under-rated Zep album, for me, it would be a tie between Presence and Led Zeppelin III.
Presence is pretty out there, even by Zeppelin's standards. Still kicks it though.
And III is the album that sort of changed how Zeppelin cut their music, so it was kind of confusing to a lot of folks who thought they knew the band after I and II. And of course, it gets sandwiched between the monster II and IV albums, which are simply some of the best albums of all-time, easily.
If pressed, I go with Presence.
But saying Zeppelin had an under-rated album is like saying Wayne Gretzky had an under-rated season in the 80s.