For Veterans developing skill lines, I vote for Rkindaleft, the Dwemer public dungeon on the east side of Wrothgar. Nice and dense with xp-juicy mob groups that spawn reasonably fast. Interesting architecture, what with the levels and high paths and all.
"Argonians have fat, scaly tails." —Rissa Manyclaws.
"Once upon a time there were three sisters: Delicious, Delightful, and Disgusting. Now, Delicious and Delightful were both very pretty girls..." —Brendalyn Jurarde.
"I smell to the nobility." —Indrasa Avani.
"A bargain with an animal is not a contract made." —Haderus Atrimus.
"Redguard makeup for sale. Free samples. Secret ingredients. Unique application method. Lots of satisfied customers." —The Mudball Goblin (aka, Cognac Vinecroft)
"Your armor looks like underwear." —Shuns-the-Knife.