If you could only play in one of these areas any time you loaded up the game...

Cyrodiil.

Vvardenfell - Too much of the island is made up of Ashlands.
Shivering Isles - A little too weird for my taste.
Solstheim - Too small.

Vvardenfell... because of its size, and its culture and architecture.

Vvardenfell...Prior to it being wiped of all life, that is.

Cyrodiil's leveling system pisses me off too much so I'd go with Vvardenfell

It's a stretch between Cyrodiil or the Shivering isles, Shivering isles has the never ending quests of defending your towns once you become madgod, but cyrodiil has the dark brotherhood, and a much bigger living breathing world.

Vvardenfell - I just like it's design and atmosphere better, though Solstheim would be a close second. Cyrodiil, at least in Oblivion's recreation of it, was bland and generic. Though perhaps if Cyrodiil had been done right, more accurate to lore and with more interesting environmental designs, I'd be more likely to favor it over Vvardenfell.

Out of those places, I guess I'd have to go with Vvardenfell, a much more creative and interesting place than Cyrodiil or Solstheim, Shivering Isles would come in second, however, it's not big enough to make a full game, and while the environments are great, the fact that everyone there is insane, though an interesting diversion from the often boring and almost always generic characters we usually get from Bethesda's games, would likely lose its novelty if I spent the whole game there.

Of course, this only applies to pre-Red Mountain eruption Vvardenfell, I'd imagine a volcanic wasteland with no one living in it would be kind of boring to explore. And really, I want to see places that are new to me, in other words, not any places I've seen in Morrowind or Oblivion or their expansions, or Daggerfall, for that matter.

I would pick Solstheim if it was larger, but since it's not, I have to go with Vvardenfell. Not that Vvardenfell is a bad thing, it's fricking awesome.

I love Cyrodiil the most.

Cyrodiil. When you're north, it's all snowy, when south it's muddy and rough and when in the middle, it's an epic forest. Vvardenfell is awesome, but there is a massive crater in the middle, it's not as colonised, and it has Cliff Racers. Cyrodiil also has the Imperial City and Bravil :3

Pretty much this. If i had to choose, i like the atmosphere, culture and lore surrounding Vvardenfell. But i would much rather have new setting to explore.

This aint gonna end well...

I vote Solstheim, of course. What would I do without my puppies?

I don't entirely understand the question. Do you mean if future Elder Scrolls games could only be set in one of the provinces we've seen? Cyrodiil.

If the question is just which game we would play if we could only choose one, Morrowind.

If this means in future games, then I have no interest in playing in Cyrodiil or the SI again, a post-apocalyptic Vvardenfell, so Solstheim populated with more Dunmer it is.

Vvardenfell if this just means the way they are now.

Ehh, that's a hard one

Cyrodill is large and diverse, but at times bland
Shivering Isles is unique, beautiful and fun to explore, but unwelcoming and sometimes I just wanna go for a walk and look at the trees
Vvardenfell is large, culture rich and has the best 'atmosphere', but feels a little deserted at times
Solstheim, well I love the tundra, but it's a little too small

I guess Cyordill cause it has a little bit of everything

Cyrodiil. Vvardenfell is too alien, barren and dark for my taste.