Text (NE+errata): Exile target creature with the greatest power. (If two or more creatures are tied for greatest power, target any one of them.) [Oracle 2009/07/08]
It can only target a creature with the greatest power. This means you pick such a creature on announcement, and if that creature is not still the one with greatest power on resolution, then this spell is countered since its target is illegal. [Nemesis FAQ 2000/02/07]
If the creature with the greatest power (or all the creatures if there are more than one) are untargetable, you can't cast this spell. [DeLaney 2000/02/21]
