Difficult one to call. For me, (slightly) obtusely, Controversy. In retro-spect it signals the whole about to blow up style and danger of his funk/pop sound, plus Annie Christian is one hell of a song. Big nods for Purple Rain (which really is his best album?) Sign O' The Times is majestic - and the mirky Black Album which beyond the songs is music myth personified. I get a feeling the vault and the YouTube block will unlock now - yet I admire how much he kept it real to the sadly too soon end.