1. Black Flag
2. AC brotherhood
3. AC 2
4. AC liberation
5. AC 3
6. AC 1
7. AC revelations
8. Unity

Have not played rouge or syndicate yet

1. Assassin's Creed
2. Assassin's Creed Brotherhood
3. Assassin's Creed Revelations
4. Assassin's Creed 2
5. Assassin's Creed 3 Liberation
6. Assassin's Creed 3
7. Assassin's Creed Bloodlines

To me the first entry was the best because of the most interesting time line and by far the best cities in the series. Also the game was short and tight and not as crowded with stupid copy and paste missions as with the successors. With the mission structure you really felt like an assassin who had to gather enough information to engage his target and not like a do-gooder who had to solve the problems of everybody he stumbles across.

Story wise the Ezio trilogy was the best the series offered.

Didn't like 3 very much. The interesting initial character got replaced by the dull Connor, the world was a big but boring forest and the towns were bland with alike looking wood and brick houses with much too broad streets.

Brotherhood
II
Revelations
III
I

my list is maybe a little odd, but I mostly played the games for the main mission and skipped almost every sidequest. I don't really know why I liked revelations so much. I think the pc port ran so smooth after a stuttering experience in 1-brotherhood. it had some features like automatically follow a person while they talk and the paths from main mission to main mission were rather short.

1. black flag
2. revelations
3. syndicate
4. assassin's creed 2
5. brotherhood
6. assassin's creed 3
7. unity
8. assassin's creed 1