What sort of experience are you looking for? I love an open world, and the experiences differ depending on what you're looking for.
Story
Elder Scrolls V: Skyrim
probably the most bang for your buck as far as open world goes. It's single player, offline like you want, but also can be modded to add nearly infinite experiences. If you want some mod suggestions, check out MxR Mods on youtube. He has a large catelog of mods for skyrim ranging for ridiculous to immersive.
Prey
The story itself in prey is rather linear, but the world itself can be explored vastly. It's a bit more contained, and does change over the course of the plot. Definitely something worth checking out but, make sure you write down how many of what is in your room each night.
The Forest
This game is all about exploration and survival, and the first real survival game on my list here. If you're looking for something that has you crafting, and fighting against the elements along with some rather eerie foes, I would suggest this title. The focus is more on the gameplay than plot line, but everything aligns itself to the story of a Father looking for his kidnapped son on a seemingly not so deserted island.|
Grand Theft Auto/Saints Row series
These games have pretty good stories, generally speaking. While GTA doesn't seem like much on the surface, the latest title has a bit of a solid plot, and the gameplay is pretty sandboxy. Saints Row is what would happen if zero cares were given about the development, and the devs wanted to push absurdity to the limit. The gameplay is still fairly fun, but both of them get stale after a few play sessions.
Gameplay
Dark Souls
Any title out of this series plays well, and is very focused on mastering the mechanics. I'd suggest 1 or 3, but 2 can be interesting as well, and each of them can be played indepent from one another. The stories exist, but are burried beneath exposition exclusively from characters telling you what's going on, and very loosely implied elements (generally). Some things make you think there's more going on here than meets the eye, but generally you're more focused on not dying to the next boss. It's rewarding to defeat and enemy that's difficult, but it is frustrating.
Ark Survival Evolved
A great openworld survival game with very little plot. Taming dinosaurs and trying to collect as many as you can just feels awesome, and the game itself can be played online as well as an option. The developers constantly update it, mod support is enormous, and the two expansions add so much to the game. Want to have a house on the back of a giant dinosaur? GO FOR IT! I love this game.
There are plenty more out there, but these are the ones I'm most experienced with. Hope it was helpful!