Which Assassin do you think best acted as the role of The Mentor? Important to note that this has NOTHING to do with character, personality or progression. Just wisdom, skil, leadership and accomplishments. I didn't add Ezio and Altair because Iknow everyone would pick the two.
Al-Mualim: He greatly strengthened the Assassin Order during a reign that lasted for at least 2 decades. He trained numerous Assassins, one of them being his successor and arguably best student. Al-Mualim set up admirable restrictions on the methods of killing such as saying no to poison, calling it a coward's tool. He pushed back various attacks by Templars, Saracens and Crusaders and kept Masyaf a safe haven for any and all citizens who wished to live in peace. All of that would have made him a legendary figure but his Templar past caught up to him and just as he betrayed the Templars years before, he betrayed the Assassins and used the Apple to forcefully bring peace to the whole world.
Ah Tabai: My personal pick out of all of them. Ah Tabai was from a lineage of Assassins. Much like Altair, he was born into the Order and dedicated his life to it. An wise and experienced leader, he would have numerous noteworthy students (Some of them would even go on to become legendary figures) such as Mary Read, Adewale and Achilles Davenport. he strictly wanted to follow the principles and teachings of Altair as closely as possible, modeling the brotherhood after Altair's during its height in Masyaf, albeit in a much more secretive light. He wasn't immune to error, though. His trust of Duncan Walpole almost guaranteed the wiping out of all Assassins in the Caribbean area and it also forced them to relocate to another location as the Templars would rigorously send troops to try and destroy the Brotherhood's main base. Nonetheless, Ah Tabai maintained the strength of the brotherhood and would pass away by 1745.
Agate: Operating as a lone cell, Agate recruited and trained Aveline and Gerald to act as his eyes and ears around the Louisiana area. His influence did not spread much but he's indirectly responsible for the wiping out of the Templar branch of the southern American Colonies through his student Aveline. He gave out many contracts and missions to free slaves around the area, which would help in spreading Assassin principles of freedom around the southern colonies. Agate's time as a slave, his best friend's betrayal and his mentor's disgrace drove Agate to be a very paranoid and jaded individual, leading to his suicide and presumably purging Assassin leadership in Louisiana.
Achilles: Trained by Ah Tabai, Achilles Davenport would go on to establish the first proper Colonial Assassin guild. He greatly strengthened Assassin presence in the 13 colonies by recruiting and training various students. He would also establish a powerful Assassin Navy that controlled the Atlantic and the Caribbean for a long time. During his tenure, the Assassins were at their strongest. That would all come crashing down during his expeditions for First Civilization artifacts. He would send Shay Cormac to one of the sites and the rest is history. These actions resulted in the complete Annihilation of his brotherhood at the hands of Shay and the Templars and spend 9 years in isolation after being spared. He would gain his redemption when Connor comes knocking at his door asking for training to combat the Templars. Through Achilles' tutelage, Connor would go on to completely destroy the Colonial rite of the Templars and Achilles would pass away redeemed.