Hey Matt, welcome!
Difficult to decide. As you mentioned, the obvious answer would be Skyrim. When it comes to the vanilla version, there are some interesting quest lines, the dynamic leveling system (while can be exploited) is pretty good, the world is huge but what makes it the "best" is the modding community.
You can make Skyrim an entirely different game if you choose to do so. So from that perspective alone, there is huge variety on what you can do with it. You can make the combat more exciting, make the game more difficult, fix some A.I. issues, populate the areas with more NPCs, roam the world in a car, ride dragons, use guns etc, the possibilities are endless.
There may be games that are better story-wise but I haven't played much of them (e.g. Pillars of Eternity).
I can't think of any other relatively recent PC game that allows you to change that much about your gameplay experience, so yeah, I'll go with Skyrim for now.