That depends on what you like and how you play. Also, if your computer is more robust, there are some amazing graphical enhancements.
Nexusmods has a lot of great mods on it (and some that suck). Here's a link to the top 100 on there: http://www.nexusmods.com/skyrim/mods/topalltime/?a...
There are around 20 that I won't play without. Rather than give you a list from me, there are two lists that are really helpful. GEMS and STEP:
GEMS (Gameplay Enhancement Mods) http://www.nexusmods.com/skyrim/mods/23826/?
STEP (Skyrim Total Enhancement Project) http://www.nexusmods.com/skyrim/mods/11/?
These are lists put together by a bunch of testers and they offer different kinds of improvements to look at. They made it easy for me to find far too many really good options.