In terms of how the actual world is made technically, then nothing quite measures up to GTAV on the market. It's safe to say that the GTAV games are pretty much always the benchmark at how you do an open world. The amount of work put into it is unparalleled. This is coming from someone that's not even a GTA fan but I just admire the level of detail put into the world. You can literally do anything you want in it. Ask anyone, on average, and they will more than likely say the exact same thing.
Now as far as actual beauty goes and which open world I find myself absorbed in the most, then that goes to Shadow of the Colossus. I put that game in for a few minutes just so I can relax my mind. The vast, gorgeous open plains are a sight to behold!
