The title above says it all. I haven't played a lot of them, but my favorites are (so far) Outcast, GTA 3, and GTA Vice City.

- The Witcher 3: Wild Hunt
- The Elder Scrolls V: Skyrim

Skyrim, Oblivion, Morrowind, Daggerfall, Saints Row 2, Saints Row 3, Fallout 3, Fallout New Vegas.
Oh and Minecraft, if it counts.

does it really count as open-world, i didn't had that impression as i was playing it?

Fallout
Fallout: New Vegas
Gothic 3
Grand Theft Auto 3
Grand Theft Auto: Vice City
Grand Theft Auto: San Andreas
Mount & Blade: 1866 (a total overhaul mod)
Mount & Blade: Warband
Postal 2 (base game only)
Risen
Saints Row 2
Saints Row: The Third
Saints Row: IV
Scarface: The World Is Yours
Sleeping Dogs
The Elder Scrolls: Skyrim
The Getaway*
The Getaway: Black Monday*
The Sims 3**
Two Worlds

*These are questionably open world, as though the entire world is open and explore-able, you're still fairly funneled through the missions with little freedom. However each has a Free Roam mode.

**To me The Sims 3 is open world. You can freely move about the city, enter and otherwise interact with many different buildings and basically traverse the entire town at your leisure.

There are others that I would list, but I haven't completed enough of the game to really say much on them. Games in this category would include Outcast, Morrowind and others.

Hmm. Terraria is included in "open-world games" right ? so.. yes. Terraria.

Unless you're talking about Tactics, it most definitely is.

The Elder Scrolls - Chapter 2: Daggerfall
GTA 2
GTA San Andreas
Saints Row: The Third
Postal 2
Ultima VII

GTA San Andreas!!! I still play it from time to time and i have finished it 100%, bothering to do even the unique stunt jumps!

Darksiders 2, if it counts as open world, with all those regions, maps and horse travelling...

And certainly, absolutely, OBVIOUSLY, that Witcher 3. Especially with new game + now and all, it has to be the BEST there ever was! With Witcher 3, Skyrim, Fallout 3/Vegas/4 and co become totally irrelevant!

Aaand... MORROWIND! I personally consider it far better than skyrim and essentially better than that horrible (again, strictly personal opinion), Oblivion...

Elder Scrolls
Might and Magic 2
Mount and Blade

I've yet to play Fallout 3 and Witcher 3, but I'm sure they'll end up high on my list.

I've been tempted to make a remake of Might and Magic 2 just for my own fun. It was so old and held back by the small amount of space a game could fit on (I think it was 8 512k floppies on my C64). It was brilliant.

You start in a town as a peon with a knowledge of your world history (the creation story and the major events that lead you to your current time -- they year 900). As the story unveils itself, you learn that you mission is impossible and your world will be destroyed. So you must gain favor with a brilliant (if not wacky) king and use his time machine to travel in the same open world during 9 different eras (you can go to the start of each 100 year segment). For the most part, the world is the same but without NPCs and with harder creatures. But there's a ruin in 900 that becomes a city in a previous year and there are major historical events you need to change in order to make your mission possible in your time period.

It could have been so much more. And today, it could be made far, far better with even just the silly tech of RPG Maker.

So, my hope of the future, is a fun RPG in an open world with time-travel elements.

I haven't managed to beat any of the Elder Scrolls games yet, as I tend to feel overwhelmed and lose interest after while, but I played through the huge Nehrim (total conversion mod for Oblivion, with 120+ hours of gameplay) and absolutely loved it. Probably because the world design was 100% handcrafted and there was no random stuff and no level scaling.

Skyrim was a wonderful enviroment and had some great random encounters.

I love fallout,2 and 3. Fantastic games all around.

Witcher 1/3 (does the original count) was fairly open ended and I loved it.

Saints row 2-4 were hilarious.

Red Dead Redemption is one of my favourite of all time. I adore the western themes and the story was fantastic.

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.

Morrowind
Terraria
Dwarf Fortress

Gothic 1, Gothic 2, Risen, Avernum, Geneforge.

Geneforge's world is broken up into sections, but they are often accessible to the as-yet-unprepared player.

I enjoy exploring regions which are beyond my nominal skills.