I'll be honest, I've only played 1, 2 and part of 3. Assassins Creed 2 was the first one I played and I feel it drew me in and kept my interest best.
The Italian backdrop is perfect and I even travelled to Florence (IRL) at one point and the game reflected that city incredibly well! I remember looking up at walls and towers and picturing how Ezio would scale them. The combat and depth are far improved from the first game and I recall the story and character development of the protagonist is far more engaging than the first.
I bet it still holds up and would be a good place to start if you're trying to save time and want to skip the original.