Quaternion.Slerp

Description

Spherically interpolates between a and b by t. The parameter t is clamped to the range [0, 1].

// Interpolates rotation between the rotations "from" and "to"
// (Choose from and to not to be the same as
// the object you attach this script to)

var from : Transform;
var to : Transform;
var speed = 0.1;
function Update () {
    transform.rotation = Quaternion.Slerp (from.rotation, to.rotation, Time.time * speed);
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Transform from;
    public Transform to;
    public float speed = 0.1F;
    void Update() {
        transform.rotation = Quaternion.Slerp(from.rotation, to.rotation, Time.time * speed);
    }
}

See Also: Lerp, SlerpUnclamped.