A personal ranking of the mainline AC games (everything universally regarded as such, plus Freedom Cry and Liberation), all of which I played in the relatively short timespan from November 2020 through January 2022. In short, I found all four eras (the classic Altair/Ezio games, the Kenway games, the Initiates games, and the Witcherized RPGs) to be a mix of good and bad. I very much played these games out of order because Unity was the first one to pique my interest and then I started jumping around the series trying to get my bearings. I know some diehards think the series needs to be played in order but, buddy, if AC1 had been my first AC game, it would have been my last AC game. I'm not too tied to the specific ranks and would more naturally put them in tiers like this:

I really love Unity and Black Flag, for very different reasons. I think they're fantastic and have really stuck with me.

Revelations, AC3, and Rogue were all solidly enjoyable (AC3 more for the sweep of the story and the main character, Rogue more for the gameplay, Revelations for a mix) and I could see myself replaying them someday.

AC2, Odyssey, and Valhalla I had a solidly good time with but can't really see myself replaying.

Brotherhood, Freedom Cry, Syndicate, and Origins I feel somewhat similarly about but less warmly.

AC1 and Liberation are the only ones I really didn't enjoy. I think AC1 is a really interesting game and I have a lot of respect for it, but I hated the experience. Liberation I hated the play of less but I also felt like there was nothing about the game to really recommend, unlike AC1.