I did a little assassin creed run this summer. Started with Unity and went to Odyssey.

Unity by far has the best story-line. The game mechanics are also more focused on being an assassin, not a brawler. Sneak and stab, drop smoke and disappear when you get in trouble.

Syndicate - they changed the focus to more brawling. However the mechanics were pretty bad and the storyline was predictable and stupid to say the least. You can still play the assasin for most of it if you want.

Origins - They went all out into brawling with this one. Complicated all of the controls. Easiest way to make it through the game is as an archer. The story lines are dull and some are just plain weird. The offer you tons of different options in gear but only a few are worth keeping. Not much chance to actually assainate anyone. Lots of weird half baked stuff like the naval warfare in the story line but nowhere else.

Odyssey - it’s basically the same as origins. They spent more time fleshing out the mechanics. Added in a ton of options but you only use a few. Added in a ton more brawling combat and very little assassination. In many instances trying to play as an assassin is just not possible. The combat is often broken. Beating the bosses is all the same. Spam arrows while dodging until they weaken, get a hit in and run away- repeat. Save often because the game crashes regularly.

Yes I played all the way through and beat the main quests and side quest on all of them.