toxicTom: It all depends what you want from an open world game.
I would say
Witcher 3 is currently the best kind of it's kind. A near perfect symbiosis of free-roaming and exploration on the one hand, and storytelling on the other. No place feels generic. Even the smallest side quest is a nicely told story, and some side quests are epic enough that other games would use them as the main quest... And the feeling of decisions having an impact on the world has probably But of course is also has its limits:
The things you can do are all tied to solving quests and killing things or people. And also there is no character creation - Geralt is Geralt.
If that bothers you
Skyrim is probably a better choice as far as fantasy open goes. You can create your own character (that could also be a cat or a lizard) and there's tons of stuff to do besides quests and fights: Craft your own bow, go hunting, grow your own vegetables and then cook a fine meal in your self-built home with your better half and kids... Or go back to
Ultima 7 where you can even knit your own pullover...
Aforementioned
Morrowind is also one of my favourites, because the pace is totally different. Early in the game you are told "You are weak and know nothing! Go get some experience and knowledge". There is no urgency to the main quest. And to understand it you actually have to work: Read books, talk to the savage tribes. Otherwise, if you just follow "the next quest" you might end up finishing the game but not really understanding what the hell is going on. Also, Morrowind has one of the most interesting game worlds ever created IMO.
As for the
GTA titles (3+): Those are rightly praised. What stops me from fully enjoying them are the mini-games - I just suck at them.
And then there are a ton of other games, like "open space" games, that could be mentioned. There I'm still looking for a game with the scale like the old
Frontier - Elite 2 or
Frontier - First Encounters. Some people really dig the X-Series for their huge universe and lots of options. I've played (and finished) only the first (fun, but repetitive) and had a glimpse on X2 (ran like shit, sadly). In
Freelancer I kind of lost all interest after the story was finished and the "real open part" began, but on the way there I had a lot of fun.
The only GTA game that had mini-games was Chinatown Wars, though I may be wrong about that. As for the Elder Scrolls, I never really got into it that much, but Skyrim seems to be pretty cool.