Best completely open world game ever?

Doesn't matter what platform or genre. Just say what you think the best one is (and when I say completely open world, I mean that games with a lot of bigish, rather constrained outdoors areas you can go to but that are completely sectioned off by loading screens, a la Dragon Age and STALKER, don't count). 
My favorite is probably Oblivion. While, yes, the world did feel very cookie cutter in spots, there was just so much of it and so much stuff to find and do, it more than made up for the clone NPCs and identical dungeons (Morrowind is a close second, but the combat and the awkward first six hours or so really dampened my opinion of it).

A Link To the Past

World of Warcraft. I can't think of any other game where you can travel absolutely anywhere, without any loading screens right from the get go.

@BaneFireLord: But Oblivion has big areas sectioned off by loading screens

Jeez. Out of all that I've played, I had the most fun with Fallout 3. Just loved how it looked banged up and torn apart. Course I'm a sucker for post-apocalyptic environments.

I would have to GTA: San Andreas. Man, that game world was amazing and so much fun!

World of Warcraft seems like the logical answer.

I would say GTA San Andreas because it was really fun to mess around in the world.

Just Cause 2! Everything happened in the open world in that, no loading screens. One of the best games I've ever played.

I have no clue what "the best" is, but my favorite is The Elder Scrolls III: Morrowind.  
 

I would have to GTA: San Andreas. Man, that game world was amazing and so much fun!

Haven't played inFamous 2 yet, but it's apparently better than the first (which I loved), so that.

The GTA games have always had some good open worldness to them. But I'd give my vote to World of Warcraft. Aside from the load screen between the continents; you can walk from south to north without any loading, and it would take forever.

Red Dead Redemption for me. I've never been a more realized game world.  

Well, as far as open world games that I had fun messing with the world in, I'd say Morrowind, and GTA 3.  
 
But, games that I had the most fun with overall would be Infamous 2, Red Dead Redemption, and Fallout 3, because I don't think I could go back and have a great experience with Morrowind or GTA 3.

Grand Theft Auto: Vice City, San Andreas is a great game, but I think its to big for its own good.

Just cause 2

WoW if MMOs count.
 
Red Dead Redemption if not.  The world in that game was just so great.

Fallout 3 because I like to explore the environment and discovering all the little details bethesda put into their games. I'm sure this will be replaced with Skyrim soon though.

FABLE 3

Red Dead Redemption. Never has a video game world felt more alive.

inFAMOUS or Fallout 3

WoW, Morrowind, Red Dead Redemption, or Fallout 3.

I thought the point of Oblivion was that the world 'loads as you go'. You only got loading screens when you travelled too quickly. Or something.
 
Anyway, terminology or none: Morrowind.

My favorite is oblivion.

Morrowind had the atmosphere, freedom and content that Oblivion utterly lacked.

inFamous and Saints Row 2 are up there with my favourites too. Guild Wars 2 will likely take the top spot for me though. The developers did an amazing job with the background lore and the world in the first game, putting all of that into a completely open setting is going to be amazing.

GTA 4 and Red Dead Redemption

@UnsavedHero said:

Jeez. Out of all that I've played, I had the most fun with Fallout 3. Just loved how it looked banged up and torn apart. Course I'm a sucker for post-apocalyptic environments.

This.

Just Cause 2! Everything happened in the open world in that, no loading screens. One of the best games I've ever played.

Elder Scrolls IV: Oblivion

You can't get more open than Morrowind.

@valrog said:

Full size image I really need to play Just Cause 2. Also, the statistics for World of Warcraft is inaccurate.

This doesn't seem accurate at all, the scale seems completely off.

And Guild Wars Nightfall definitely wasn't an open world game. It had some pretty big areas, but they were all sealed off and mostly linear. It looks like they've based the whole thing on the game art and dodgy figures.

@valrog said:

Full size image I really need to play Just Cause 2. Also, the statistics for World of Warcraft is inaccurate.

This doesn't seem accurate at all, the scale seems completely off.

And Guild Wars Nightfall definitely wasn't an open world game. It had some pretty big areas, but they were all sealed off and mostly linear. It looks like they've based the whole thing on the game art and dodgy figures.

Just Cause 2!

I've never spent so much time in a game world as I did in Oblivion, so my vote goes to that.

Also while it's not open world, I have to say Ocarina of Time has one of my favorite world's to spend time in.

Open fields, towns, mountains, lakes, a desert, a horse to ride around in, and I could even go fishing! I spent so many hours fishing...

By a long shot morrowind

Runner ups

Fallout 3

Fallout new vegas

oblivion

just cause 2

Mount and Blade series. Sid Meier's: Pirates.

Morrowind, by far.

Funny. I actually thought of this the latest days when I have been playing Infamous 2. I came to the realization that Infamous is the only sandbox game where I actually completed both games in the franchise and did so with much joy. I did not like Prototype, Oblivion was the game I bought an Xbox 360 for but regretted it greatly soon after (the choice of palying Oblivion, not buing the console), Assassin's Creed does the same thing over and over again and I have gotten tired of it, Grand Theft Auto 4 lost me halfway and Crackdown 2 was horrible and pretty much the same stuff.

The Funny part is that Infamous only has a few new feutures. The big difference is that I actually want to play more of the same thing. When I began playing the game I though to myself "Hmm...okey...this seems to be very much like the first game, I think I will get tired of this." Instead my motivation playing the game grew with my powers. Therefore I have come to the conclusion that I must like Infamous most of all sandbox franchises the most.

Red Dead Redemption was great.

Fallout 3. No contest.