Who has to show first?

[DeuceKicker]

For some reason, it seems B&M players would rather lose a kidney than expose their cards at the showdown. They'll wait and wait and wait, point at each other, look at the board, look at each other impatiently, and wait some more, while all the time the dealer (and everyone else at the table) is saying, "Flip 'em over!" They're waiting for the other player(s) to show so they can muck a loser or flamboyantly slam down their winning hands.

Not that it matters to these players, because they wouldn't listen anyway, but what is the rule about who has to show first: Is it the first player left of the button, or is it the last player to have initiated a bet/raise?