I've gotta go with Ronnie and Steve from Lynyrd Skynyrd. Steve Gaines' only studio album with Skynyrd was Street Survivors, and he absolutely kicked that band in the ass. Everyone had to elevate their game to keep up with him, and it was some of Van Zant's best songwriting.
I also wonder how Skynyrd would have progressed with no plane crash and if Allen Collins hadn't faced so many demons. He went through some hell after the crash (his wife died during a miscarriage), and he ended up falling deeper into drugs and alcohol. He crashed his car while driving drunk, killing his girlfriend and paralyzing himself. He never played guitar again, and died just a few years after the Skynyrd reunion tour in '87. Would loved to see how that band would have progressed with Rossington, Collins, and Gaines all being together for years. I'd have loved to see Ed King come to that mix also.
"Well, if you can’t have a great season, at least ruin somebody else’s." - Olin Buchanan