Unity Ads supports all licensed versions of Unity since Unity 4.3. We always recommend using the latest release, or patch release, version of Unity. We generally advise against using Unity Ads with beta releases of Unity since they can be unstable.
If you are using Unity 5.2 or later, it is strongly recommended that you use the in-Editor version of Unity Ads through the Services window in Unity.
If, for whatever reason, you prefer to not use the Services window, you can still use the latest Unity Ads asset package instead, available for free through the Unity Asset Store. The Unity Ads asset package supports Unity 4.3 and later.
In any case, it is always recommended to use the latest version of Unity Ads.
Unity is not a requirement for using Unity Ads. If you are using a different engine or framework, you can still use the Unity Ads SDK.