I have been replaying the older games and this is my personal ranking. I concentrated on the main quests and didn't do much of the Ubibloat. I did do what I considered the best sidequests like Leonardo's weapons and all the tombs in the Ezio trilogy - and Homestead in AC 3.
Brotherhood
Still the best Assassin's Creed game, not much to dislike tbh. Well, except maybe the inability to crouch. The story was great and all the mechanics (hiding in crowds, assassin's squads, building and earning money) are my favorites. Tombs are the best they have even been in the series.
Unity
Unity kept the most of the things that were great in Brotherhood. Paris is imo the best location in the game series and it is so alive. Rifts, climbing the Eiffel tower, was just awesome and felt so fresh. Parkour in Unity is the best in the series, just so fluid and plain fun. Also I liked the combat as it took a step back from the killing machine that Ezio was in Revelations. Return to stealth was welcomed. The story was also fine, sometimes even good, despite being pretty typical revenge story which had been seen in the series few times already.Brotherhood wins only because of the superior tomb design and more inventive missions.
AC 2
The first of the trilogy is surprisingly strong entry even today. It starts a bit badly when there's a lot of combat mechanics missing but once it gets started, it can be really, really good. Parkour is not very precise and can be damn right frustrating (like in many other AC games though even more so). But I love Ezio, he's really likable character, story is good and tombs are entertaining.
Revelations
To my surprise I hadn't played this before even though I thought I'll be replaying it. In ended up liking it quite a lot. Sure, it doesn't offer much new and mechanics are starting to feel a bit old after the first two games of the trilogy. Bombs are... well, I didn't use them much except for the usual smoke bombs. What I loved though was Ezio, again, seeing him as an older and more somber man. His whole arc was done really well. One of the best parts was playing as Altair, those sequences tied the story arcs beautifully together and they were really well done too. Loved the spectacle of the game too.
AC 3
Better than I remembered. I think, when I first played this, I did too much of the side stuff which made me bored of the game. Now that I played only the main quest, I think it is considerably better. Homestead missions and the whole community building was a great idea though it takes probably a bit too long, there's too many quests, especially as quite a many of those are pretty mediocre flower pickings. The whole trading/crafting thing works well but I think it is ready too late in the game - like I couldn''t craft a lot of stuff until in sequence 9 maybe but that's only three sequences from the end. Modern day stuff is the best in the series. Connor is a bit bland. Oh, and I hate, hate, hate ship combat. (Which is, btw, the reason I never played Blag Flag longer than maybe an hour.)
Odyssey
Not really an AC game, tbh. I hated the rpg trappings but liked Cassandra as a character. Story was honestly pretty bad and went out with a whimper. The world was far too big as it is now in Ubisoft games, moving from one place to another takes ages. The whole end game stuff is the modern Ubisoft bs, wasn't interested in it at all, I despise grinding, especially in AC games where I feel it doesn't belong. Also, ships.
Syndicate
Siblings were great, London was boring. For some reason Syndicate lost the personality Unity had. I didn't like the story and the mission design felt really uninspired. I had to force myself to finish this one and I did it only because of the rather entertaining main characters.
Origins
Didn't even finish this. It is beautiful, yeah. Bayek is a good character as is his wife but the story was all over the place and the game systematically forced me to grind and do silly side quests which I didn't want to. Also those mythological enemies in a game series which had had it's roots in historical realism was just stupid.