In case you missed out on Assassin's Creed 2 when it came out in 2009, now might be your biggest chance as Ubisoft has put up the game free for PC. It's available to download in the Ubisoft store, right now.
For the unawares, Assassin's Creed 2 addressed several concerns from the first game, expanding on the world, creating stronger dialogue, improved exploration, and developed one of the series' most memorable characters, Enzio Auditore da Firenze, who would later star on two more Assassin's Creed games - the much beloved Brotherhood, and the, less beloved, Revelations.
Assassin's Creed 2 is still considered to be one of the best games in the series 12-and-still-going run. It pretty much setup the groundwork for future titles, always expanding the world and adding new gameplay mechanics every now and then.