The one who called has to show last is the rule we always use. So in this case, UTG called on the turn so button has to show first. However button can muck, then noone has to show and UTG just scoops the pot. I was talking to a friend about this yesterday. He said he once mucked a bluff in a big tournament, but regretted this. It's much more valuable to see what they're calling your bluffs with than to not show whatever bluff you ended up on the river with.