OptimalHit

This mod is focused on the PvP aspect of minecraft.

The mod draws 3 boxes onto other players. First 2 (referred to as Green and Red) show the best vertical area while the last one (referred to as Red2) shows the best horizontal area to aim at.

The mod's configuration screen can be accessed by these commands:
/optimalhit
/optimalhits
/ohit
/optimalh

You can configure the following:
Enable/Disable the mod
Enable/Disable any of the 3 highlight boxes
Size of any of the 3 highlight boxes
Box opacity
Whether to only draw on Players
Whether to highlight hitbox or collisionbox

From personal use, I recommend the default settings.

It uses the same technology as F3+B (Collision boxes), therefore it does not show players through walls neither does it show invisible players.

(Made for survival mode, creative mode reach works a little bit differently)

(Highlight example on a villager)