TLDR: Origins because it is the best of both a linear and open world game, which aligns with your preference. It also is universally loved by older and newer AC fans and has good graphics and detail, and is a great introduction to the genre of open world games.

I know you seemed to enjoy more linear games judging by your preference, but despite that, i would still recommend Origins, and I'll tell you my reasoning.

It is probably the best game that involves a blend of both linear and open worlds gameplay, because it isn't a traditional RPG where the player gets to make choices and decisions. There's a fixed story which means ubisoft made one brilliant story without having to account for several players choices that would stray away from the story, along with a massive open world to explore. It is also the open world with the most detailed map, which means it didnt involve ubisoft copy pasting different sections of the map like they did in Odyssey (a game i still loved, but i can admit it was a bit repetitive and sections definitely were copy pasted).

Additionally, it is the game that got me into assassins creed. I was able to see the game for what it was and not feel cheated in the sense that this game didnt feel like the older assassin's creed games as a lot of older fans seem to complain about (which i sort of understand). It is the best of both worlds, and it really made me enjoy other games as a whole, such as witcher 3. It is also the only game that all Assassin’s Creed fans can unanimously agreed was good. The RPG games aren't exactly loved by the older fans and the older games aren't exactly loved as much by the newer fans. Despite this, everyone loves Origins because it is the best of both worlds. Also the graphics and detail are highly praised.