The Escapist has published a short, gallery-style article entitled 8 Sandbox Games That Will Leave You Wandering. And Ultima 1 is the very first game listed thereupon:

Richard Garriott’s now classic Ultima I: The First Age of Darkness is considered by many to be the first sandbox video game. What Ultima I allowed you to do was to experience the world on your own terms, you could go wherever you pleased and encounter whatever you discovered over there.

And, of course, the successive entries in the Ultima series continued to improve upon and refine that openness. But certainly, Ultima 1 provided the foundation upon which that game design paradigm was built. And arguably, the very idea of open world gameplay owes much — if not everything — to this earliest Ultima title.