I don't remember some of those games too well anymore, but I think I used to have the impression that Revelations was the best one in terms of quality. Sure, it was more of the same, but it did things better than the previous games. The third game felt a bit, uh, cold, and I didn't really like Black Flag as an Assassin's Creed game. I haven't played Rogue (yet, despite owning it) because I consider it a bit of a spin-off so it's not very high on my backlog, but I suspect it's in the same league with Black Flag.
The first game is quite interesting though. It made a huge impression on me, and I still dislike the lighter tone the series took after the first game. Still, it was quite flawed in many ways, and when we're discussing the best game, it doesn't really deserve to be too high on the list. The only other Assassin's Creed game to have impressed me was Origins (which obviously is off-topic here), so I don't hold even the series's best games of the 7th generation in such a high regard because while good, there wasn't really much to make them special. But for the purpose of this topic, my answer is still Revelations.
