FWIW (nothing I suspect):

Elvis: great in his early days with Sun, went downhill rapidly after that. Great voice though. Sad he wasted his talent, and his life for that matter.

Buddy: great songwriter and records, in spite of any perceived limitations in his voice or musicianship. He was an innovator.

Eddie: great songwriter and records, great guitarist too, good voice for the stuff he did.

All three were influential beyond their own lifetimes. The difference is that Eddie and Buddy died in their prime and will always be remembered for the short period where they burned white hot. If Elvis had died similarly he might have been remembered the same way - sadly many people remember him for decline into the bloated parody of himself he became before he died (also young but not as young as the others). Me, I like to think of him in the Sun days.