Probably the top 4 guys that are 'legendary' (to me) yet still never saw their full potential?
Stevie Ray Vaughan: I can't imagine how much great work we could have seen from him if he hadn't stepped onto that helicopter.
Randy Rhoads: Same as SRV, if only he hadn't have gotten onto that plane.
Greg Allman
Buddy Holly
I don't think I could list any as the "greatest", however, my favorite in the bunch would easily be Stevie Ray Vaughan.
Now, onto some preaching from me... I do NOT consider any of the musicians that died due to a shitty lifestyle (booze, drugs, etc) to have died "tragically". Moon, Bonham, Bon Scott, Janis, Morrison, Jimi, etc... all of these folks died due to self-induced lifestyle choices. That's not tragic (by my personal definition). Folks like Cliff Burton, Selena, John Lennon, Aaliyah, Marvin Gaye, Dimebag Darrell, etc..... these guys (as well as the ones I listed above) were taken from us either via murder or some other accident. Some probably had already hit their peak, others had not.
Being a strung-out junkie who died of an OD, well, so sorry, so sad, but it's not tragic and if they didn't die that day, they could have just as easily died the following day, week, month, etc. So, for that reason, I do not and will not include any in any list I ever make on this topic.