Favourite: Storm Coast and Hissing Wastes.
Storm Coast was a great and atmospheric area, in my opinion. Looking for the Grey Wardens signs of passing and reading the journals left behind was very interesting. The battle between the dragon and the giant came completely unexpected on me, so when I saw that, I was like: "Wow, cool!"
Not to mention, I wasn't aware before release that there were giants in DAI, so seeing that huge creature in the distance, slowly coming out of the mist was a rather terrifying sight.
In addition, I love dwarven lore, so finding all them ruins left behind made me curious.
I just happen to like the concept of a creepy coast I guess.
The Hissing Wastes were also great, since the dwarven tombs there added some interesting new lore. It was the only of the desert areas that felt as open as a desert area should be, in my opinion. Not feeling restricted was great, since it really made the desert feel vast.
Of the non-open areas, I thought that the Arbor Wilds was beautiful and that the place in Cassandra's companion quest looked absolutely gorgeous. The Winter Palace was majestetic and unlike anything i've ever seen in a game before. It's also the location of my favourite main mission.
Least favourite: Hinterlands.
I did not enjoy the Hinterlands as much as I would have liked to. The area is beautiful, but there is so much uninteresting content there, and it lacked an engaging storyline. Still, I did like seeing Redcliffe again.