The Witcher 3... it's barley a competition, take that out however and it's hard to choose between GTA5, Horizon, MGS5 and Red dead. I'd also add God of war in there as it's full open world, with linear segments yes but still full open world. I'd add in BoTW but I was severely let down with that game, still deserves to be along side them never the less as the fun I had with it was due to the open world. Skyrim ..maybe but It felt dated on release and the setting was bad for traversal, quite annoying actually but Oblivion and F3 are up there.
Nothing comes close to the witcher 3 though, it has transended so many genres and aspects of gaming.