I don't know if "impressive" is the best word to describe The Witcher 3. I mean, you don't get to fly the skies, create or destroy the landscape like in other open / semi-open world games.

What you get is a candid, warm personal journey.

How did they manage to nail both -The game mechanics- and -the story telling elements-? Well, the game benefits greatly from the books and lore in general, something that other games have to struggle with. This could work as a disadvantage as well, but the developers managed to build an innovative, unique and organic experience that never felt forced.

I don't know if The Witcher 3 will pass the test of time as well as Skyrim did, but I can say that Geralt and his friends will stay with me longer that many other AAA's out there.

GTA V. True open world of an ironically accurate depiction of Los Angeles.

Honurable mention to red faction Guerilla which was also completely open world and the destruction was the most detailed I have ever seen, still to this day. Also Mercenaries 2 because you could call a nuke over the city and wipe out a few blocks and it was unscripted.

Never got around to playing any just cause games but number 3 has really caught my attention.

Yakuza series, they've restrained me from beating up innocent civilians.

Definitely Dragon Quest VIII: Journey of the Cursed King. Such a beautiful world.

Xenoblade for the verticality. However, I have not had exposure to many open worlds.

Gonna have to say Witcher 3. The world is massive with a ton to do, yet it is able to deliver a rich and deep storyline that the previous games delivered (which were only semi open world).

This to me is a huge achievement cause while an open world game can have a good story, the level of depth in the plot is hardly on par with well crafted semi open games or even more linear story based titles. Witcher 3 however manages to pull this off however, and does it very well.  It has its flaws, but it is truly a masterpiece on a technical and emersive level.

Gothic 3 by far.
Its still the most beautiful and atmospheric game world ever created. Everything is handcrafted and usefully placed.
The quest-design is easy and brilliant so you dont need navigation arrows and ugly HUD advisers.
Thats what most modern RPGs suffer from the most, you just cant enjoy the world because immersion gets killed by the Ubisoft like world designs.
Even Witcher 3 has these problems, the quest design isnt made for playing without minimap and markers =(
With the community Patch 1.75 Gothic 3 its still one of the best oldschool RPGs out there, everyone should try it.
https://youtu.be/LJ3dDdezMms

Right now Morrowind GotYE (two official expansions, plus I added the official add-on mission of Firemoth siege) and Gothic II + Night of the Raven expansion, but I expect them to be beaten by some games in my backlog (Skyrim, the first two games in the Witcher series and Mount & Blade; my system isn't ready yet for the Witcher 3 and I'll wait for the Enhanced Edition anyway, so its unlikely that TW3 makes me update my list sooner than next year), if they'll do, I'll post an update.

Skyrim did it for me, but Witcher 3 got my attention and I'm looking forward to buying it.