Skyward sword, . lol jk xD I would choose xenoblade Chronicles,.

In a sense of  fun and variety, I have to give it to GTA San Andreas over GTA V.
If it's about detail, depth and scale, I'll give it to Far Cry 3, Fallout 3 and Red Dead Redemption.

I would say the Witcher 3

Morrowind and Skyrim for me :)

Red Dead Redemption was my first real open world game and it's still my favorite.

Bloodborne

Fallout 3 or New Vegas are by the most comprehensive and fun open world games in term of quality. the Elder Scrolls series is solid too (obviously with the same publisher) but it does not seem to maintain the quality and curiousity of intrigue across all quests and maps. I've noticed that, for example, the world of Skyrim looks the same wherever you are much more than, say, Fallout 3. granted one is in an apocalyptic mutated surrounding so that gives it a bit of an advantage in terms of creativity

some of the games people have mentioned aren't really 'open world' just saying

I'd have to give it to GTA V and this is coming from someone who disapproves of the series sometimes ...

Far Cry 3 and Fallout 3 were both groundbreaking and amazing too ...

Gothic 3.