Ultima 6: The False Prophet was released many years ago by Origin Systems, and it is the game I hold in the highest regard to this day. You can pass it in as little as half an hour if you know what you're doing, or you can spend hours upon hours wandering Britannia in pursuit of each and every quest. It offers an open world, with building interiors on the same map as their exteriors, and also features truly labyrinthine dungeons that interconnect...and even (in some cases) end you up on the opposite side of the world.
And the story of Ultima 6 is pretty great, as well. It starts out as what seems to be a fairly straightforward "defeat the monsters" quest...and then subverts that trope handily, with a keen mid-game twist. And it's a rarity amongst RPGs for how little combat is actually required to pass it: I can finish the game with only three monster kills, and I could probably do it in less than that if I really put my mind to it.
Of course, Ultima 6 is a game from a bygone age. As a 2D RPG produced for MS-DOS, it boasts a native resolution of 320x200 or so, and supports a whopping 256 colours. To my eyes, it looks fine...but not everyone agrees. Which is why the Ultima 6 Project is so awesome: it re-creates and expands upon the story of Ultima 6 using the Dungeon Siege engine. Which, granted, isn't the newest game engine going, but it's still a fine 3D engine that supports an open world experience.
The Ultima 6 Project loads as a module for Dungeon Siege, although it also overhauls elements of the game's menu and interface; it's almost (but not quite) a total conversion, really. You can find download links for it at the the project website, or grab it from my website. It will obviously require a copy of Dungeon Siege to play...and that is where things can get a little tricky, because Dungeon Siege doesn't always play nice with modern versions of Windows.
To install Duneon Siege on Windows XP or Windows 7, you will probably have to make use of a "No-CD" crack of some sort. I won't link to a site where one can be found, but they are out there. On Windows 8, I found that the Steam version of the game -- which comes bundled with Dungeon Siege 3 (which, by the way, is quite a good game in and of itself) -- ran just fine, and also worked handily with the Ultima 6 Project mod.
This project has been over a decade in the making, and in fact has seen previous releases that have been well-received. This version, 1.1, will be the final release of the module, and it's the one I would recommend trying...because you deserve to experience the story of Ultima 6.
Just don't expect to be able to pass this version of it in half an hour.