The biggest component for what makes this a must play is the story. AC2 follows Ezio Auditore da Firenze from birth to his induction into the assassin order. Ezio is a likable protagonist partially due to walking in his shoes from day one and partially due to his many flaws, such as his love of women and quick temper. AC2 builds up Ezio's relationship with his family members early on and then forces the player and Ezio to bear witness to the death of his father and brothers. Ezio's vow of vengeance easily becomes the player's. Ezio's targets are personal, which made taking them down extremely satisfying. As the story progresses through the years, Ezio grows, changing from the scared boy who lost his family to a man who understood the bigger picture. This is an extremely satisfying evolution to witness and makes AC2 one of the best origin stories ever told.
On the other side of things is the story of Desmond, which has a lot of light shed on it in this installment. He's ostensibly reliving Ezio's life as a means to receive assassin training, but in reality it's to get to a message buried in the middle of Ezio's life, which adds context to the modern day plot and the series as a whole. Meanwhile, this entry possessed a lot of opportunities to learn more about the ancient rivalry between the Assassins and the Templar order. Glyphs left by the mysterious Subject 16 were challenging puzzles that expanded upon what the pieces of Eden were and their role in establishing history as well as the famous historical figures who wielded them as either Templars or Assassins. And for every Glyph unlocked was another piece of "The Truth", a short video that when completed, changes everything about the world and provides a starting point to the madness that's followed for millenia.
While the story is undoubtedly the reason to play this game, the gameplay isn't bad itself, providing a varied sets of missions and some fun toys to play with. The game is the start of more linear Assassin's Creed games, which while constricting in some ways, kept the game from getting stale. The setting is also well worth noting. Venice and Florence look amazing and are recreated in detail that made immersion into this world very easy, which in many ways is what makes Assassin's Creed II a brilliant game, the setting, the story, and the characters all come together to tell a tale of growing up and all the joys, struggles, and knowledge that comes with it.