Randy Rhoads - He took the Van Halen style, which obviously was in itself ground breaking, to new levels. The first Ozzy album is truly a great piece of art that honestly blew away everything Sabbath did. And I like Sabbath a lot. Not sure how he would have evolved from Ozzy, but it sure seemed like it could. Hard rock fell into a cocoon of shitty hair metal that would go away until G'N'R.
Stevie Ray Vaughn - He wasn't all that young, but he was hitting his stride and was becoming transformative when he died.
Steve Gaines/Ronnie Van Zant - Gaines brought fresh ideas and blood into Lynyrd Skynyrd and helped make their best album to date right before he died. Van Zant could make southern rebellious music both soulful and heartfelt, without ever being sappy. Southern rock never recovered. Street survivors showed that Gaines and Van Zant could have led it down a path that never got taken..