Hriston
Dungeon Master of Middle-earth (He/him)
Use a ruler or some yarn, or a laser pointer, don't physically draw them, just quickly count them.
That's OK. I like drawing lines.

Which one to hit? the one with the highest AC that the roll hits. Ties go to closest to the firer.
Interesting suggestion, although it seems a bit counterintuitive to have the projectile hit the highest AC that it can automatically considering the dex bonus and whatnot that goes into that. It almost makes it seem like the projectile is making an intelligent choice to maximize its effect based on what it can feasibly do.
Also breaking ties by proximity would lead to the problem of having to impose disadvantage on the attack if the intervening creature were within 5 feet, potentially changing the result that lead to the target being selected in the first place.
How about this: If the missile attack misses the target by less than or equal to the AC bonus due to cover, then the creature who is covering the most lines gets hit. Ties are resolved randomly based on size because a larger creature takes up more of the space that the stray shot is moving through, and so is more likely to be hit.