If you want to understand the AC plot, you'd get AC1.
We have AC1 and we're hoping to get hands on ACB.
>_< I hate not understanding a plot.

ACII was also great and I definitely recommend it. Though you will have to deal with ACB spoiling you a bit more as ACII doesn't have many of the features that ACB does. But it has a great story and it will reveal more to you. To get AC's storyline, ACI really isn't too necessary. I really liked ACI probably for a few different reasons. I got it when it first came out, and at that time, it was a technical miracle. It was so fresh and original. No game had done what it did. I also loved the philosophy aspect of the game (something the newer ones are lacking, but oh well). The setting was also my favorite so far, as it was set during the dying days of the third crusade when the Templar Order was at its most powerful. I still wish they had let us kill Conrad de Montferrat as he is the most well known victim of the Assassins in real life. Instead, we killed his father, William, in the game.