Thank you! Also, not the ruling I expected, I thought it would be either both or none, not pick one.
To be clear, in this example it would be the one who controls KotRH that would choose, not the one who controls the the strongest creatures in question?
Presumably this decision can be made (and changed) each time it applies? So in effect it means all of them for something like KotRH, whilst a spell that said 'kill the strongest creature' would just get the one nominated at that moment.