The original Assassin
The story of Altair is the one that kicked off the entire franchise. This vast universe unfolded by telling his dark turn from apprentice to master, which evidently took us the furthest back in time to 1191 AD, set during the Third Crusade.
Not only was this setting the most fitting one for the general theme, but it also laid the foundation for the series as a whole, both in terms of its lore and the actual mechanics. Sequels built upon what the original Assassin’s Creed presented, and every character that followed was connected to Altair in some way, albeit in terms of ancestry or by a certain item or piece of equipment linking them together.
It’s a traditional story of a student uncovering the truth about his beliefs, causing the lore of Assassin’s Creed to take a dark, historic tone. Whatever you may think of the original game in terms of its mechanics or general structure, there’s no denying that it caused a tremor in the gaming industry that can still be felt to this day, and Altair is mostly to thank for that.