It's hard to make any sense of that kind of list. To me a good game is a game that you want to play again, sometime after you discovered it, and aged well. It's hard with AC game since every game fixes and improves lots of things that makes it hard to play previous game (just try to go back to AC3 after Unity or even AC2 after Unity, it's a nightmare since it's not responsive)
Assassin's Creed brought the core game design. A pure experience that is still playable today because it works. The narration, a redemption quest about faith, loyalty and doubt, is amazing, and fits the gameplay. The repetition of the missions and same objectives has to be seen with this perspective : the more you kill, the more you get powerful, the greater the danger is and the more doubt with his faith Altair has, the more you ask yourselves questions as a player. You can't enjoy the game if you don't feel that. Also, the 3 major cities with the Mayssaf village and the Kingdom area are great hostile playgrounds. That's "raw" but you can't criticize how the last games are "too big" and criticize when Ubisoft kept it simple with key ideas and coherence.
Assassin's Creed II shifted the game into a more linear, cinematic-revenge story with more generic gameplay. It aged very badly. The controls will make you want to smash your gamepad and the cities lost their magic. It's not really fun anymore.
Assassin's Brotherhood and Revelations expanded that formula and the story. Interesting, fun, but I don't want to play them again. Especially after re-trying AC2.
So the Ezio trilogy was a great adventure between 2009 and 2011. It isn't really anymore. Revelations had some interesting ideas thought, with new setting (cool city), a very different Ezio, the Altaïr memories and some narrative tricks that worked.
Assassin's Creed III tried to renew the franchise with better combat, less linear and urban, and great narrative techniques (the prologue!). The father/son relation was greatly written from the beginning. A new leaf that had its flaws —mission structure mainly — but the combat was so much fun, the story so deep and the exotic elements (boat, village building, hunting etc) really added something that bond the different parts of the game together. A fresh point of view.
Assassin's Creed Black Flag brought a fantastic and coherent adventure from both a narrative/story and gameplay perspectives. An even bigger adventure than AC3 was. A massive living open-world with the right amount of things to do and a need to explore all the areas.
Assassin's Creed Unity tried one again to renew the franchise, this time by coming back to the origins with big ambitions. Sadly, the story and narration are the worst from the main games with AC:Brotherhood and they fucked up too many things to let you enjoy everything. I can't forgive the treatment they gave to the historical events, even though Paris is magnifique with their new engine. It added a lot, maybe too much, it lacked a good production and creative direction. Such a waste.
I need to replay it to confirm my thoughts because all the negativity around the game killed pretty much everything positive you could think about it.
Didn't finish Rogue and Liberation yet but they wouldn't really fit into a list. Didn't play Syndicate at all but from what I hear, read, watch it has everything that I like (coherence, identity, key ideas, atmosphere, narrative innovation).
So I'd rank them like this
Assassin's Creed (2007)
Assassin's Creed Black Flag (2013)
Assassin's Creed III (2012)
Assassin's Creed Unity (2014) tied with Assassin's Creed Revelations (2011)
Assassin's Creed II (2009) tied with Assassin's Creed Brotherhood (2010)
So my top 3 is basically 3 unique point of views of a time period and a setting that succeeded at bringing something fresh — even with its flaws —, a lido-narrative coherence, and that's why they aged better than the Ezio trilogy and will probably age better than Unity (and Syndicate?). That's the best formula and that's how they should chose the setting/time period in the next games. If not, that'll be another Unity : a game that lacks identity because they didn't quite identify the French Revolution and Paris, or didn't simplify enough their ideas (just cut 1/3 of the game content, from coop to useless side missions, put that resources-people-money into important things).
Also, special prize to the Black Flag DLC called Freedom Cry because of how he used the tools from AC4 in order to build an engaging story about slavery and maroons people, with two or three key ideas that were — once again — coherent and that you can clearly remember.
