マウスで回転するやつ（1. EventSystemをヒエラルキーに追加 / 2. このコンポーネントを回転したいオブジェクトにアタッチ / 3. 同オブジェクトにIsTriggerなコライダーをアタッチ）

MouseRotator.cs

using UnityEngine;
using UnityEngine.EventSystems;

/// <summary>
/// Rotates the GameObject by mouse drag.
/// The X and Y axes correspond to yaw and pitch.
/// Requires a collider attached on the same object and an EventSystem in the hierarchy.
/// </summary>
public class MouseRotator : MonoBehaviour, IPointerClickHandler, IPointerDownHandler, IPointerUpHandler
{

    public Transform Camera;
    public float Speed = 720f;
    public float CtrlMultiplier = 4f;
    public float AltMultiplier = 0.25f;

    Quaternion InitialRotation;
    bool IsMouseDown = false;
    Vector3 LastMousePos;

    void Start()
    {
        InitialRotation = transform.localRotation;
    }

    public void OnPointerClick(PointerEventData e)
    {
        if (e.clickCount == 2) transform.localRotation = InitialRotation;
    }

    public void OnPointerDown(PointerEventData e)
    {
        LastMousePos = Input.mousePosition;
        IsMouseDown = true;
    }

    public void OnPointerUp(PointerEventData e)
    {
        IsMouseDown = false;
    }

    void Update()
    {
        if (!IsMouseDown) return;

        var m = Input.mousePosition;
        var dx = LastMousePos.x - m.x;
        var dy = LastMousePos.y - m.y;
        var r = Screen.currentResolution;
        var s = Speed / new Vector2(r.width, r.height).magnitude;
        if (Input.GetKey(KeyCode.LeftAlt) || Input.GetKey(KeyCode.RightAlt)) s *= AltMultiplier;
        if (Input.GetKey(KeyCode.LeftControl) || Input.GetKey(KeyCode.RightControl)) s *= CtrlMultiplier;

        var axis = new Vector3(-dy, dx, 0) * s;
        axis = Camera.TransformDirection(axis);
        transform.Rotate(axis, Space.World);

        LastMousePos = m;
    }

}