I might just be seen as crazy for thinking this but I say Frank Zappa.
"Freak Out!" was Frank's debut album and it is arguable the first concept album, one of the first double albums (let alone the first debut double album), and it was a heavy influence on "Sgt. Pepper", which is often cited as the "greatest" and/or the most "influential" album of all time.
On top of that Frank's work made a hug impact on heavy metal music as we know it by inspiring Black Sabbath (specifically Tony Iommi) and Alice Cooper.
He also influenced various other critically acclaimed and highly original acts like The Red Hot Chili Peppers (mostly in John Frusciante) and Primus. As well as funk music icon George Clinton and the epitome of musical parody Weird Al.
Think about that for a second....Frank Zappa inspired perhaps the most important heavy metal guitarist of all time (Iommi), arguably the most important funk musician of all time (George Clinton) , and the highest selling polka and comedy act of all time (Weird Al). Those are some pretty far stretches from each other. Frank was a true innovator.