Speaking of mods, what different ones do you recommend?

I've been playing on the PS3 for a while, greatly enjoying my Orc barbarian with his dual axes and my dunmer nightingale, but the time has come to switch over to PC, mainly because I really want to start modding.

I have some great memories from my days with a modded Oblivion, and a quick look at the Nexus shows some promise. What would you say is the most important mods for skyrim in the following categories?

Textures/environment
AI
New items
Quests
Locations
NPC:s
Magic
Other