It is hard to pick between Dark Souls and Skyrim. I have platinumed Dark Souls and I am well over 200hrs in Skyrim atm.
The really awesome thing about Dark Souls is once you load the game, you are done with load screens unless you fast travel. To have an open world game that is really huge and streamed so well with little taking you out of the experience is an incredible feat.
On the other hand Skyrim is more than just a story placed in the world. Many developers create games and simply build the world and NPC's designed to drive that story from beginning to the end. But with Skyrim there are other things happening and to those people the bigger story you are going through doesn't really matter to them or of no urgency. That makes the world as a whole more believable.
Based on scope and what is actually pulled off, I would give it to skyrim. I think it is easy to see a game that massive go the wrong direction real fast, but it doesn't, it is an impressive game for any platform.