Hmmm, well let's see.
The unofficial patches fix a lot of bugs.
Sounds of Skyrim series
The Places of Skyrim mods
Immersive Armors pack (adds various well done lore friendly armors)
Chesko's Wearable lanterns
Chesko's lore based loading screens
Lanterns of Skyrim (all in one)
Prometheus' Beast Skeletons
Paarthurnax fix (not sure if you know the ending already, but if you don't, just trust me.. it's nice to have this choice)
Any mod that adds arrow smithing (smithing extended or arrowsmith)
Dance Of Death killmove mod
Light Armor Hoods pack
Dragonbone weapons complete
Psychosteve's Dragonpriest masks
aMidianborn's Book Of Silence adds a lot of quality retextures of the vanilla armors
One of my favorites, the real
Fus Ro Dah (adds the fus ro dah from the Skyrim trailer. There's two versions, one is with the chant and the other without).
I could go on for a while listing mods
There's a lot of really good ones out there. Just browse through the Steam Workshop's best rated of all time as well as the top files at the Skyrim Nexus and pick out what fits you. I personally won't use a mod unless it's lore friendly. Be sure to check if there's compatibility issues with certain mods (any mod that uses the same area or that edit similar things are usually incompatible, most new weapons and armors don't have many compatibility issues).
I think there's issues with having too many mods due to the memory limits of Skyrim, just so you know. Although, I have around 130 different mods and I haven't run into any problems.