Okay, I didn't know that.

Another possibility would be Cannith.

I know not much about the Challenges, but one gets I think 2 free Challenge Tokens each day.

The group difficulties are said to be bugged, though : The bigger the group, the bigger the challenge. That additionally to the overall quest difficulty level.

And some elementals of the Mansion challenges have become infamous, especially the air/water elementals, because of their difficulty.

http://ddowiki.com/page/Vaults_of_the_Artificers

http://compendium.ddo.com/wiki/Challenges