Fallout 3 - Theres no denying that Fallout 3 is (imo) Bethesdas masterpiece. They really got everything right, from the brooding atmosphere, world design, difficulty, and sense of survival. Been playing Fallout 3 since its release, and I have NEVER got bored of it.
Oblivion - Oblivion was the game which told gamers around the world 'the next gen of gaming has arrived'. After LOTR... lots of movies attempted to cash in on the big battle spectactle, (300, Troy etc..) but none came close. Just as after Oblivion, lots of game companies tried to muscle in on the open world genre.. (two worlds anyone?) But none could best Oblivion, the original. From the excellent atmosphere to the beautifal landscapes, its no suprise people are still rolling new characters in Oblivion to this day, simply to walk around the countryside doing nothing but admire its beauty. Main quest? sorry I was watching the sunrise.
Morrowind - Would have pipped in above Oblivion, but despite being a huge lover of MW, it just doesn't have the timelessness of Oblivion. A fantastic first attempt at world building, a world which was as unforgiving to the noob as it was rewarding to those with the patience and intelligence to learn its ways.
Skyrim - With a library of open world games under their belt, Beth really knew what they were doing when it came to Skyrim, and it shows. A living, breathing world which if judged for this alone, would have smite the competition. Unfortunatly, too much was cut which highlighted in fact how it was not so much the world which defined Beth's games.. but the customisation which was available in it. In other words, I feel that in Oblivion or MW, any fantasy archetype I can imagine in my head, I can create and play in their game, which isnt as much the case in Skyrim. This more than other tends to want you to do things a certain way, such as *spoilers* forcing you to become a werewolf for a certain guild, forcing you to start the dragon attacks for the civil war, forcing you to join the college for the MQ.... and many other instances or sort. The fact remains forcing the player into something that directly contradicts their chosen playstyle is sort of sacriledge in these sorts of games.
I would give mention to games previous to MW, but they are likely to fall on deaf ears.