Good question. I'm sure we'll all land on the same restaurant/answer.
The Spouse almost had a conniption answering this as the landscape is littered with tried, tested and true ribeye tastings but, ultimately, the prize goes to the Cowboy Ribeye at "Flying Fishbone".
I know: how does a restaurant that has "fish" in its title make the best steak on the island?!? Ahhhh... the culinary mysteries of Aruba continue...