I've 100%'ed Origins, Odyssey and just recently Valhalla. Taken me over 600 hours combined.
The world of Origins is superbly portrayed and I feel that it has the best individual textures. The story is also good and I really like Bayek and little touches like his voice being muffled if you cover his mouth etc. Odyssey has a far larger gameworld, which is both a good thing and a bad thing but has some additional game mechanics which arent present in Origins. On the flip side the lack of a shield bugs me.
If I could only have 1 of them, I'd go with Odyssey because I have a strong interest in the ancient history of Greece and a lot of familiarity with its locations and myths.
Have you considered getting the Ubisoft+ pass? That way you would get the Ultimate versions of Origins, Odyssey and Valhalla and all their DLCs