Shannara was for me, and surely for many others about my age, the entrance to the world(s) of High Fantasy. At the age of 9 or so, my mother handed me a copy of The Sword of Shannara: a massive tome that dwarfed anything I’d read up to that point. (Let’s be honest, it still dwarfs almost anything anyone ever reads!) I was captivated, and I still am, two decades and a half-dozen or more readings later.
The Shannara series, especially the original trilogy, is a microcosm of the world of published Fantasy. In Sword, we get a story that starts as a mirror image of The Lord of the Rings. By the end of the book, the story is very much its own, though, and when we get to Elfstones, we see the author really stretching his wings, taking his story in new and very interesting directions. Finally, in Wishsong, Brooks is on his own. The story is darker, more personal, and more philosophically weighted than its predecessors. Similar is the journey that Fantasy at large has taken since Rings.
Arguments can be made about other authors or books being superior to Brooks or Shannara. But I doubt whether anyone outside Tolkien himself has done more than Brooks to make and keep Fantasy relevant, readable, and sellable (the Del Reys are certainly in his debt!). If other authors have surpassed him in some respects, then they have done so almost certainly thanks to his influence. Readers and writers of fantasy owe Terry Brooks much thanks.