Unless you particularly have a fondness for Dark Brotherhood or Thieves Guild, definitely Orsinium.

Orsinium. Great. Just great. Huge area, dungeons, trials, public dungeons, quests, dailies, worldbosses...
This is one of the best zones. A must have for orc lovers. Highly recommended.

If you like stealing, sneaking, heists and such, Thieves guild is good for you. Area is a bit small, but there are a few dungeons and a new trial if you are into those. TG was a nice update IMHO, but nothing you really need.
Repeatable quests send you around the world. Main quest is entertaining. Easy gold for the moraly flexible.
New mechanics for hiding, trespassing, bounty, thieves troves,...

Dark brotherhood is for murderer. Literally. Some achievements are like "murder 20 innocents in kvatch/anvil". If you like blood and brutal kill animations, take it. Area is small, no trials, 2 dungeons. For me it feels like an Addon to Thives guild, but not to ESO. The murder quest "black sacraments" are copies of thieve guild heists. Only you have to murder someone.
In some side quests you meet "old friends" from previous zones. Some are fun, some are annoying. Die Raz! DIE!
For me this one was not so entertaining, mostly because of pointless mass murder.

Orsinium if you want casual solo PvE (Wrothgar) or endgame solo PvE (Maelstrom).

Imperial City if you like endgame group PvE (ICP/WGT dungeons).