This document lists 59 game settings (GMSTs) that affect detection mechanics in Skyrim, and explains their function.

Large increase to AI visual detection ability both indoors and outdoors. Over-powered for most setups, but ideal for some like ELFX Enhancer/Hardcore and RAID Weathers. It's what I use - "The way it's meant to be played".

Large increase to indoor visual detection ability, Medium increase to outdoor detection. Over-powered for most setups, but ideal for ELFX Enhancer combined with vanilla weather or some other weather mods.

Small increase to AI detection ability. This is what vanilla detection should have been. Suitable for most setups if you want a bit more realism but not much more difficulty.

Medium increase to indoor visual detection ability, large increase to outdoor detection. Over-powered for most setups, but ideal for ELFX Hardcore + RAID Weathers (if high/high feels too hard indoors).

Medium increase to AI visual detection ability both indoors and outdoors. Suitable for vanilla lighting and many other lighting/weather mods if you want much more realistic and challenging sneak gameplay.