1 - Fallout 4
2 - Fallout New Vegas
3 - Saint's Row the Third
4 - Skyrim
5 - Far Cry 4

Fallout 4 is a clear #1 for me but everything else is relatively close together. Fallout 3 and Far Cry 3 could easily be up there, as well.

Borderlands series would be up there but I consider it semi-open.

1. Red Dead Redemption
2. Witcher 3
3. Metal Gear Solid: TPP
4. Fallout 3
5. The Saboteur

(GTAV, Fallout:NV, Skyrim, Far Cry 2, Just Cause 2, Ass Creed 2, Burnout Paradise easily could have been my list)

Editing my original list, as I thought this only inluded Eighth Gen.

1.- Skyrim
2.- Witcher 3
3.- Xenoblade Chronicles X
4.- Bloodborne.  
5.- Dragon Age Inquisition.

1 - Bloodborne
2 - Far Cry 4
3 - MGS V
4 - The Witcher 3
5 - Infamous 2

The first place is undisputed. The others could be ranked differently depending of my mood.

1. Oblivion
2. Morrowind
3. Skyrim
4. Dark Souls
5. Fallout: New Vegas

I wouldn't really call the Souls games open world, especially the original Dark Souls. Open world games are usually ones where you could access just about the entire map from the start of the game if you so desired. Dark Souls/Bloodborne's a bit more like Metroidvania style exploration.

1- The Witcher 3
2- Red Dead Redemption
3- Skyrim
4- Batman Arkham City
5- Bloodborne

1. Assassin's Creed II
2. Red Dead Redemption
3. Assassin's Creed Brotherhood
4. Dragon Quest VI
5. Infamous

1. Red Dead Redemption.
2. Mafia The City of Lost heaven.
3. GTA Vice City.
4. The Witcher 3 Wild Hunt.
5. Assassins Creed 2.