There is no definitive answer to this question as it largely depends on personal opinion. However, some achievements that may be considered rare include those that are only attainable by a small number of people or those that are extremely difficult to obtain. For example, the 'Secret Achievement' in 'Halo 3' is only unlocked by 0.01% of players and requires completing a very challenging set of tasks. Similarly, the 'Platinum Trophy' in 'Uncharted 4: A Thief's End' can only be earned by 1% of players and is notoriously difficult to obtain due to its many requirements. Other rare achievements may not be as statistically uncommon or challenging, but still hold a high level of rarity due to their subject matter. For instance, the 'Achievement Hunter: Griefing Specialist' in 'Gears of War 3' can only be unlocked by killing 100 teammates in competitive multiplayer matches - something that most gamers would consider highly unethical (and therefore unlikely) behavior. In conclusion, there is no single rarest Xbox achievement as rarity is relative; however, there are certainly some accomplishments that are more difficult or less common than others.