Most people just have the monster move where it is best for the party. I know some people have the monster be "intelligent" in this case and if there's a spot that better for the monster they move it themselves. A die is also reasonable if you want to go to the trouble.
I've seen a variant where the Normal Monsters move to the worst place (best for players) and Elites move to the best place. Fun idea but might be hard to decide what the best square is.