Mathf.Atan2

Description

Returns the angle in radians whose Tan is y/x.

Return value is the angle between the x-axis and a 2D vector starting at zero and terminating at (x,y).

Note: This function takes account of the cases where x is zero and returns the correct angle rather than throwing a division by zero exception.

// Usually you use transform.LookAt for this.
// But this can give you more control over the angle

var target : Transform;

function Update () {
    var relative : Vector3  = transform.InverseTransformPoint(target.position);
    var angle : float = Mathf.Atan2(relative.x, relative.z) * Mathf.Rad2Deg;
    transform.Rotate (0, angle, 0);
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Transform target;
    void Update() {
        Vector3 relative = transform.InverseTransformPoint(target.position);
        float angle = Mathf.Atan2(relative.x, relative.z) * Mathf.Rad2Deg;
        transform.Rotate(0, angle, 0);
    }
}