If an opponent has multiple creatures and/or planeswalkers tied for the greatest mana value, that player chooses which one to sacrifice.

Starting with the next opponent in turn order (or, if you cast Flare of Malice on an opponent's turn, starting with the opponent whose turn it is) and proceeding in turn order, each opponent chooses a creature or planeswalker with the greatest mana value among creatures and planeswalkers they control to sacrifice. Then those permanents are sacrificed at the same time.