My nominee for most mind-bending chord in Prince's whole catalog: the Bb he goes to on "wish" in "Sometimes It Snows in April." The song is a pretty straightforward (and heartrending) ballad in E and the chorus has a very soothing Amaj7 feel...until he sings an F natural over a Bb chord on "wish" (1:35 below). It feels wrong in the best way, almost cubist, and I guess you could say it suggests the profound disorientation of grief and/or the impossibility of a wish for eternal love. Anyway, it hits me like a quiet thunderbolt and gives me a little shiver every time I hear it (and try to sing along with it). Facebook post with comments here.