From the 3, AC1 was the most difficult, mostly cos u can't heal. The last fight against Robert and his men was the most difficult for me. Too bad u can't use the animus after the ending any more to collect and do any undone stuff.
AC2 was great, lot of stuff to do, but not that hard, sometimes i was even looking forward to the last fights and when the game would end lol.

The Assassin Tombs and Templar Lairs were my favorite. Finished this game 100%.
ACB was the coolest, the economy system was great, but the game was too short imo. I was kinda surprised to see i was already in the last fight

. But it's a shame u can't exit the animus after the ending, which is obvious cos of the ending, but i didn't check all the stuff i should have checked to understand the story more, like the emails, but i didn't know where i could read them anyways

. I also didn't do The Truth stuff. I wonder if i would still hear the voices of Rebecca and the ppl when i did them now.
