I don't like superlatives when it comes to these kinds of questions. To name only one you have to leave out many other great ones. That said, the first one that comes to my mind is The Allman Brothers, with Duane Allman. After he died they just weren't as good. I don't think there will ever be another slide guitar player like him. Gregg Allman is a great singer and every member of that band was great but the Allman Brothers without Duane is kind of like The Doors without Jim Morrison or Lynyrd Skynyrd without Ronnie Van Zant. They can still make good music but it's just missing something.