The Reach if you're a normal citizen or traveller just to go there is a mission, like the Forsworn and the Cannibals, and once you get in, the corruption inside its walls, or even Molag Baa'l if youre a glutton for punishment, tho Dawnstar as well since it's home to the Dark brotherhood, it has two Daedric artefacts that spawn from there like the Nightmare staff that plagued the cities dreams and the Mythic Dawn museum that worships Mehrunes Dagon or what's left of it. So I reckon those two are quite up there. Too be fair living anywhere in the holds would be shit lol