Levels.cs

const int
	kPreLevelScenes = 2, // Number of scenes before the first level (splash, menu, etc.)
	kPostLevelScenes = 0; // Number of scenes after last level (score, credits, whatever)
const string kLevelScorePrefix = "Score for level ";

// ...

void OnLevelBeat ()
{
	if (Application.loadedLevel < Application.levelCount - kPostLevelScenes - 1)
	{
		Application.LoadLevel (Application.loadedLevel + 1);
	}
	else
	{
		Debug.Log ("You win!");
	}
}

// ...

int GetScore (int level)
{
	return PlayerPrefs.GetInt (kLevelScorePrefix + level, 0);
}

void SetScore (int level, int score)
{
	PlayerPrefs.SetInt (kLevelScorePrefix + level, score);
}

int Level
{
	get
	{
		return Application.loadedLevel - kPreLevelScenes;
	}
}

// ...

void OnHighscoreGUI ()
{
	int levelCount = Application.levelCount - kPreLevelScenes - kPostLevelScenes;
	for (int level = 0; level < levelCount; ++level)
	{
		GUILayout.Label (string.Format (
			"Level {0}{1}: {2}",
			level + 1,
			level == Level ? " (current)" : "",
			GetScore (level)
		));
	}
}