What is the hardest D&D creature?

The Tarrasque is the most powerful creature in the current edition of Dungeons & Dragons and it represents the ultimate challenge for many players. 20-May-2018

https://screenrant.com/dungeons-and-dragons-powerful-weak-monsters-ranked/#:~:text=The%20Tarrasque%20is%20the%20most,ultimate%20challenge%20for%20many%20players.