Download SuperTurrets AI Targeting System FREE Unity. 

SuperTurrets is a complete targeting system ready for all kind of situations as RTS games, tower defence games or mouse-controlled tanks and it is compatible with 3D and 2D games.

This package comes with an AI for aiming and selecting targets and it is quite configurable allowing you change a lot of parameters as visibility precision levels, enemy tags or configure the selecting priorities allowing you to get more complex behaviours than traditional turret targeting components.