20. Reaper - Mass Effect 3
Part of what made Mass Effect 3 such a great game was the constant sense of familiarity it had throughout. The heart of the story, the feel of the characters, yes, it was a sequel, but every element of it felt like it made sense to anyone who had played the first instalment. Nothing felt needlessly tacked on.
So, when Shepherd squared up to face a Reaper, something that was strongly hinted it in the first game but never actually seen, players were given both a sense of completion and fear. Remember that terrifying thing you heard about in the first game, well it's here now and you've got to fight it. Oh, and it just happens to be packing an instant kill death ray, for those of you who enjoy a challenge.