Video game sequels are so notorious for often sucking that many fans have dubbed the issue "sequelitus".
Be it that developers were too gung ho about rushing out another game, or just that it didn't quite have the thought and care put into it, the second instalment in a series is fraught with danger as the sort of make or break point. If it works, it can become a franchise that lasts who knows how long - if not it can find itself in the depths of a Gamestop bargain bin within the end of the calendar year.
Sometimes, however, the moon and star align, and we're presented with something that is not only better than the first game, but also arguably redeems the first. This doesn't always mean that the initial game was bad, just that it had flaws that may have stopped it from reaching its full potential - until it's new and improved sequel showed up to sort things out.
If some games suffer from the curse of sequelitus, then these are the angelic equivalents of Holy Water. Or just really good games - your pick.