If we're talking rankings:
Revelations = Rogue = Syndicate > III = IV > Unity > II > I > Brotherhood
And whew, putting those games in order like that still feels so reductive and unfair to me. Because as much as I dislike Brotherhood's writing (Vittoria agli Assassini will forever haunt my dreams), it had an incredible map, wonderful side missions and some of the best moments in the entire franchise. I was very much a rough diamond kind of game that grew on me over time. Without II and its massive revamp of the original formula, I probably wouldn't be here talking about this series right now, because really did not enjoy I on my first playthrough.
Unity was the exact correct path for the series to take and my biggest worry for Origins is still that they'll try to distance themselves from it too much and it'll, in long term, greatly hurt the franchise as a result (essentially repeating the mistakes they've made after Désilets left). IV might be, "objectively", the absolute peak of the series and III spazzed out in a hundred different directions and fumbled most of them, but the story eventually became my unquestioned favorite in the series.
And the top three? I really can't decide between them at all. Three games I might prefer over the others for their qualities, yet they still come short in certain ways when compared to the games I ranked lower. Three games I just have to put on the same level, as they all scratch a very different itch. AC just doesn't have a clear stand-out favorite for me. I can't even do a clear descending ranking like with any other franchise here. Ask me about Fallout, Mass Effect, Star Wars, whatever you want and I'll dump an order without any hesitation. Here, it's much more difficult.
I guess AC just doesn't have its GTA IV or MGS3 or whatever. All of the games had significant problems that prevented them from truly shining. At the same time they all have a consistent level of quality. There are no stinkers here either. Some games fell flat in certain areas, but other aspects of them made up for the drawbacks. Some games are far better rounded in comparison. They all provide a similar level of enjoyment though.
It might be a sign of homogenity of the franchise, but at the same time, when I look at the top three I see three very different games which utilize the same basic formula for very different results. Yes, ACs might be made of the same broth and always deliver what you expect of them, no more no less, but they are more diverse and varied than people perhaps give them a credit for, while staying consistently good.
In short, Assassin's Creed is the MCU of video games.