Texure mods:
The Official HD patch looks better than anything you can download for the static environment, IMHO.
A Quality World Map - With Roads makes everything easier.
Bellyaches Animal and Creature Pack - For much better animals.
SMIM - Stating Mesh Improvement Mod
XCE Character Enchantment - a must.
Realistic Smoke And Embers.
W.A.T.E.R.

AI:
Deadly Combat.
SkyTEST - Realistic behavior for wild animals.

Other:
SKY UI combined with Better Sorting - Another must.
Weapon And armor Fixed combined with Smithing Perk Overhaul.
Path Of Shadows - Better sneaking behavior.
Project Reality - Better weather and light.
Light Up The Night - All NPCs has torches.
Lanterns Of Skyrim
UFO - Ultimate follower Overhaul.
Less Touchy Lockpicking - To make lockpicking easier, on par with the XBOX version.