I'll probably remember this one mostly for its amazing visuals and atmosphere. The monuments, buildings (outside and inside) and streets looked great and full of details, but more impressive yet was the amount of pedestrians you could see walking around, going about their lives, rioting... The crowds really contributed a lot to making the city a believable and lively Paris during the Revolution. Similar to what I recently thought about London in Syndicate, this was a remarkable recreation of a very interesting setting for an Assassin's Creed game.
That said, it was also a fun game to play. Nothing extraordinary or original, it wasn't much different to previous games when it came to the gameplay mechanics (although the combination of level design and movement system may be one of my favorites), but it ran fine and I didn't experience any major bugs (Unity is infamous for its horrible state at launch in that regard), the story was entertaining, I liked the main characters and, as I mentioned, the environments made even just moving from mission to mission quite memorable.
All in all, a good entry in the series.
More info: https://en.wikipedia.org/wiki/Assassin%27s_Creed_Unity