Not counting vampires and daedras since every place has them.
In my opinion i would say black marsh because of the swamp leviathans, how about you guys?

Black Marsh or Valenwood.

Black Marshes terrain is extremely inhospitable, and the native wildlife appears to take extreme delight on tearing travelers throats out. More so than the rest of Tamriel at least. I would also rank Morrowind and Valenwood to be close seconds, although I wouldn't be surprised if anything horrific was lurking down in Elsweyr.

Summerset, by far, they have the thalmor.

Flame shield, activate!

Hammerfell, I'd wager. In Black Marsh you need to worry less about the fauna and more about the flora.

Black Marsh. Because swamp leviathans.

/thread

Summerset Isle. Damn elves.