For arrow shots: Wolfskull Cavern. There is absolutely no way to argue against this. Dedicated marksmen can literally clear everything except the scripted boss from a platform right at the entrance, but you have to be able to hit targets that are just the very slight bump of the top of a head over a wall at the absolute limit of arrow range to do it. It's the most beautiful place in the world for archers.
For sneaking I don't know if it's the best, but Goldenglow Estate is quite good. Lots of rooms with multiple entrances, mazey hallways, several floors, mixes of seated and patroling mercs, and a couple of fire traps. It's situated in a way that makes it very hard to get every kill as a backstab, but not quite impossible.
Last edit: (probably a lie, but oh well) I have to leave behind followers for the true sneaky adventures. There's just no way to reliably keep them quiet, no matter how much you load them with muffle boots, sneak gear, and mods that keep them from engaging in combat as soon as they smell a bandit's fart.