Definitely John Cazale.
He died at the age of 45, but his short filmography was flawless. The only full-length movies he appeared in are bona fide classics.
Chances are that if he had lived a bit longer, he would appear in movies less good, but I also think he died right before his big mainstream breakthrough. If he had only lived, say, ten more years he would probably be concidered a legend.
Shannon Hoon from Blind Melon - perhaps
Dave “Stage” Williams from the Drowning Pool -they weren’t huge but they were definitely riding high.
Jean Harlow
Darrell Kyle & Josh Hancock (pitchers for the St. Louis Cardinals)
Sam Cooke
Keith Whitley
Every full length movie he was in was nominated for best picture Oscar.
Aaliyah. She’d finished filming her role in the movie Queen of the Damned, was scheduled to begin filming for the role of Zee in the Matrix sequel and had just released her third album when she was killed in a plane crash, aged 22.
Paul McCartney. 
W.A. Mozart. His final year was one of his most financially rewarding, likely earning him the equivalent of over a half-million dollars.
This is the one.
I’d nominate Ian Curtis, too.
Biggie and Tupac
H. Beam Piper, who committed suicide, not knowing that several of his works had recently sold (his agent had died before notifying him of the sales).
Phil Hartman
There’s a weird Urban Legend that goes Miller was actually captured and killed in a Paris brothel, and the army covered it up to avoid embarrassment. His Wikipedia entry even mentions it.
Meanwhile, another example for the OP is Adolf Hitler. (Well, he DID paint some pictures.)
Jeff Buckley was only 30 and about to release a second album when he died. I think he qualifies.
And despite those 5 films getting a combined total of 14 acting nominations, not a single one was for him. 
Nitpick: Randy Rhoads.
I beg to differ; Burton was a great bass player, but you could hardly ever hear him play, even on studio albums. It wasn’t until the Black Album (with Jason Newstead) that they started featuring bass guitar as anything other than fill-in.
Nah. No Rain was released as a single from the first album in 1993, and their second album was already a flop when Hoon died in 1995.
John Bonham died just a year after Led Zeppelin headlined the Knebworth Festival, playing to crowds of over 100,000- and they’d played the biggest non-festival concert ever only three years earlier at the Pontiac Silverdome.
I’m surprised nobody mentioned Freddie Mercury; Innuendo and Greatest Hits II were #1 and #2 on the UK album chart when he died, IIRC, and Innuendo was the band’s second most successful release in the US.
Elliott Smith was polishing up his self-described “White” album, what was to be the double-disc* From A Basement On The Hill*, when he died. Some may argue his most commercially successful releases were behind him (XO), but artistically, he was as creative and inventive as he’d ever been, with an ever-expanding repertoire of sounds and musical ideas, and I personally rank some of the songs on that last album among his best.
I’m not sure Jeff Buckley should count, though we’ll never know. I’d count him as being on the upward trajectory, not having reached his peak yet.
Infomercials are art now?
Oh, he was a wonderful writer! Every time I reread one of his books I wind up going through them all and cursing the shortness of the list.
You can beg to differ all you want, but I still think Cliff Burton died at the peak of his success. I’m not sure what you’re differing about: Metallica was just breaking big when Burton died, and he was responsible for writing many of the songs for the band. You say you can’t hear him, but I heard him loud and clear on “Anesthesia,” “Orion,” “For Whom the Bell Tolls,” etc. He was a major creative force that drove Metallica to its success, but died before he could really enjoy it. He more than qualifies for mention in this thread.