There are 3 greats of open world action-adventure games this generation that stand above all others (and from which all others derive):
Kingdom Come: Deliverance - For the weight of the consequences & a meaningful world which you can affect, and perhaps the most immersive RPG ever made
The Witcher 3 - Overall and for its best-in-class writing
Assassin's Creed Odyssey - As a package and for a lighter & more fun experience focused on combat, together with an incredible setting & just so much to do
Expecting anyone on this forum to say a Sony game isn't the best that has ever been done or ever imagined, it's just a fool's errand. In reality both HZD & GoT are lite versions of this gen's AC, but really the technical limitations of the console should disqualify them both from discussion (at least until recently for HZD). I just don't understand people who like fighting the camera and FOVs of 50 with characters taking up a 3rd of the screen. Crazy!
Sadly I don't have as much faith in Valhalla but that's okay, I'm hoping next-gen AC will be Rome & they'll go nuts.