Bethesda. It's nothing to do with Skyrim, I'd have said the same even before it came out, or was even announced. TES games (as well as F3 and FNV, though the latter is not really theirs) have the largest-feeling games (Morrowind in particular, due to lack of fast travel), while offering the very liberating, distinctly open-world gameplay elements that go with it. Everything about the game screams open world and freedom (aside from the locked in dialogues, but even that's been remedied some with Skyrim, though not completely) and unlike the other games on the list like GTA (SA in particular) and JC2, which have huge, possibly bigger worlds (I believe JC2 is one of the largest worlds in gaming, bigger than the TES ones somehow [might not count dungeon sizes in that though]), they're not so much about exploration, which is, for me, the greatest thing about open world games.
Don't get me wrong, other games/studios/franchises aren't too far off. GTA is immensely liberating and the modes of travel are fantastic (particularly SA), some of the same goes for SR2 (I found SR3 underwhelming in that aspect, could be just due to lack of gameplay in it though). Prototype is another good open world game, though it's all about gameplay. JC2 is like if Prototype fucked GTA, you've got all the GTA there, but still got the hook-hand to grab people/choppers/cars with
