This, for me, is an easy one - John Bonham. The first to Zeppelin LPs are great, classic, those that followed had some great moments and some ok moments. Robert Plant was busy looking like Roger Daltry and sounding like Janis Joplin, he was good, but not so great as the totality of the band. Jimmy Page was a great studio guitarist but not so great at all live . . . if you listen to his solos, often the climax comes with Bonham's drums not anything Page is doing on the guitar - seriously, listen to their ever-popular 'Stairway to Heaven' and tell me that the highlight, the big finish to Page's guitar solo isn't when Bonham's drums come clashing in. John Paul Jones fleshed-out a lot of what could be counted the Led Zeppelin sound, and was great.
But, easily and far and away, John Bonham stood out and above the rest of the band. Look at The Who; Keith Moon was huge and so much a part of the overall sound of the band, but he was not head-and-shoulders above the others, and so after his death the band continued doing great work. After cream Clapton went on to Blind faith, Derek & The Dominos, and then a solo career, even The Stones went on after the death of Brain Jones (they went on to suck, but they went on) - when John Bonham died there simply was no more Led Zeppelin . . . he was what made that band different and recognizable and a bit stunning.