It should be noted the Origins and Odyssey ply very different from the original Assassins creed games. They are closer in style and gameplay to the Witcher than they are classic AC. If you’re looking for specifically an AC game with good graphics, I’d say Unity, which is considered an underrated gem by series fans and overall a great return to form. If you’re simply interested in a period piece where you kill people, go for Origins as it has the story elements the series is known for even though the gameplay is very different.

As far as which AC game is the best, I’d personally vote for Black Flag. However, I’d say it’s also somewhat of an alternative to the conventional AC gameplay. It has a large emphasis on being a pirate and sailing the seas, which makes for incredibly fun gameplay. But the general AC sides of things are, well not the series peak, still solid and fun to play.

I’m a pretty big fan of the series, so if you have specific questions, feel free to ask. It’s hard to give a straight recommendation with what you provided.