Just in my generation alone (2000's) we've seen some great players fall from grace and become shells of them former selves whether it be because of injuries, personal problems, ect.
When i think before my time i think of guys like John Drew and Shawn Kemp.
But for me, there has been no greater fall than Gilbert Arenas. From 04-07 the guy was unstoppable. 28 and 6 with a couple 60 and 50 point games thrown in there. It's unfortunate that injuries and personal problems have taken their toll on Gilbert because he was so fun to watch. He's still only 30 so i hope he can turn it around. But i think at the very best he's a back up point guard at this juncture in his career.
Others that come to mind from this era are McGrady, Iverson, Vince Carter, Elton Brand, Steve Francis. I know i'm missing some.