This was the year open world games really hit their stride. Witcher 3 really showed what story-driven open worlds could do, especially with how well-written the side-quests could be. At the same time, Xenoblade Chronicles X set a standard for world design that I still don't think has been topped to this day. The environments and wildlife are still absolutely stunning 8 years later, and I've never experienced anything like the change to exploration you get halfway through when you unlock the skell mechs. Even Breath of the Wild and Tears of the Kingdom can't compare with the sheer wonder of exploring Mira and running or flying past Godzilla-sized sauropods, despite them improving in so many other ways like the physics systems.
Witcher 3 is the overall better game thanks to its story and got my vote, but as an experience Xenoblade Chronicles X is in a league of its own and I think hugely underrated.