I don't know if Unity helped improve the franchise. It have gotten so much crap for it's bad launch.
Bugs, performance issues and ingame real money micro transactions (fuck you Ubisoft)
That game was a disaster. Even though the gameplay was good, it's not enough to save it in my opinion. (note: i only played the first 2½-3 hours of it on the PC... but that was plenty to make me not give a shit)
Also i heard that the coop was a disappointment. Though the new improved parkour is definitely a welcome feature.
Can't speak for Rogue because i didn't play it, but black flag sure was what fans needed to see when it came out. After the not so great (not bad ether) assassins creed 3, black flag really lifted the franchise back up and more so.
Here is my top 5 list of the games. (excluding AC1 and rogue. Can't speak for rogue and haven't played much of AC1. Also it's a long time ago. so pretty much the only 5 games i can speak of other than Unity)
- AC2 Brotherhoods
- AC2 (the original)
- AC2 Revelations
- Black flag
And then if you will Unity at number 6