Right now I feel it's got to be Deepwood Redoubt (just the Vale part, though). I was there on Saturday, and my initial plan was to sneak up to the Briarheart (using Muffle and Invisibility), but that failed when he somehow detected me. Took away 33% of my health in one hit. And I ran straight to Hag's End (which was where my "target" was).
After re-emerging to the Vale, I called Odahviing - but he didn't last very long, and once all the Forsworn were finally dead, I had to throw a couple dual-cast Heal Others at him so he could fly (or so it seemed).
The freaking Briarheart followed me through Hag's End, by the way. I had no idea he could even do that! Nasty surprise.
So yeah, Deepwood is horrible. At least on high levels, when most of the Forsworn are Ravagers and the Briarheart dual-wields hitting like a train.
As for inside areas, Forelhost still holds the #1 spot. Tolfdir decided to send me there a few days ago... Muffle and Invisibility were life-savers (most likely literally). The end room held four Deathlords, along with the easy-compared-to-its-minions boss - boy was I glad I'd unlocked "Master of the Mind". Nothing better than Deathlords Disarming and killing each other for me.