I always asked myself that question. And as many do, I also lean towards Page. He put the band together and he was the one who had his instrument next to him at all time. Plant, JPJ and Bonzo lived their lives, but Pages whole life at that time was Led Zeppelin.
But even so, without all four guys, Led Zeppelin wouldn't be Led Zeppelin. One of the reasons why they said that they broke up is because of Bonzos death. Without him, just like every other member, there is no
Zeppelin. They were all amazing.
Jimmy Page is known by everyone by his Stairway solo and the whole song in general. Whenever I tell people I play guitar, it's always "oh can you play "Stairway to Heaven?"
Paul Jones' "No Quarter" was the peak of Zeppelins every gig when it came to the light show and the whole feel of it. That song is somehow relaxing. It's just "WOW!". One of my favorite Zeppelin songs.
Robert Plant was an amazing vocalist (he lost some of the awesome rasp and hight, lets face it the guy's in his mid 60's), many singers believe that "Whole Lotta Love" is one of the toughest songs to sing. He is known for writing lyrics for most of Zeppelin songs (which, by the way many of 'em don't rhyme yet he pulled it off).
And Bonzo was one of the three best drummers who lived at that time (Bonzo, Moon, and Ginger). For some reason his 4 sec ending solo in "Rock n Roll" never gets old.
They were a molecule and now that they separated, each one of them single atoms, none of them are doing as good as when they were Zeppelin. Plant has his stuff which is neat, JPJ and Page aren't much heard of. Aside from Zeppelin fans, not many know that Jimmy Page is coming out with a new album (most of which is instrumentals, I believe) and that JPJ is writing an opera.
I lean towards Page being the most important when it comes to the bands existences and in my opinion he was the most talented of the four, but without each of the bands members, Led Zeppelin wouldn't have existed.
Again, that's my opinion.