Well my all-time favourite is ACB, but you might want to go in the order they were released.

The first one was sort of glitchy but did a very good job on ambience as well as story.

The second introduced the game mechanics that are so widely recognized as an "Assassin's Creed", plus it has the most areas to explore out of all of the games.

ACB included my beloved Recruits, which totally changed the dynamics of the game to give you a sense of wondrous power.

ACR focused more one expanding the story, but there were some new experimental mechanics as well.

Finally, AC3 revamped the gameplay style of the series and attempted to make the game more accessible for controllers whilst creating a more fluid set of moves for KB M users. It hails from the mechanics of AC1 2 and is very glitchy in and of itself.

Hope this helps!