John Rutsey
Even though he kind of followed his own path. He was more of a straight forward rock drummer, but liked to party and had diabetes.
A blessing in disguise to the band, their "new" drummer is pretty darn good..
Don't know if Rutsey was unlucky, in that I cannot conceive of Rush achieving what they did musically and commercially if he had remained in the band rather than them recruiting the "new" guy.
A few others I would put in the same category (ie not unlucky, but rather, if they had remained in the lineup, the band would not have achieved what they did.)
Dianno being replaced by Dickinson.
Rod Evans makes way for Gillan (Deep Purple - the 3rd studio album is my favourite in the band's entire discography, but they would never have achieved what they did as a band if Evans had remained vocalist).
Kaye replaced by Wakeman (I cannot imagine Yes recording Fragile and CttE without Wakeman's input)
Peter Banks makes way for Steve Howe
Mayhew gets the chop from Genesis, who go on to replace him with Collins.