Here, I just use a macro with the Targetenemy command on my mouse wheel button. It always targets the closest thing unless there's something within a few yards behind me.
It works like this:
TAB with no existing target: Always starts with closest, then steps outward until furthest in range, then returns to closest.
TAB with existing target: Steps outward from there.
Targetenemy command: Always picks closest, even if there's an existing target further out. (This works over a very wide arc, I'd estimate 270 degrees, and is not limited by cone target mode unfortunately, so it tends to catch closer targets to your side.
A good place to try this is on a row of practice dummies. The ones close to the Kholusia arrival point are easy to get to if you're in the Crystarium
