Least favorite? Black Marsh
Largest Province? Without looking I want to say Hammerfell
Largest ESO Province? Ebonheart
Most people?...do you mean highest population in theory, or highest population in regards to playerbase? In thoery, I would say Aldmeri has the highest population, in regards to playerbase I'm going to guess Ebonheart.
Personally, I would love the next TES game to take place in Alinor or Valenwood. Overthrow the Thalmor and strengthen the Aldmeri Dominion in a new light that encourages people to join rather than submit or die.