Gosh - There are heaps of good mods.
For landscape (depending on your system) There is the Landscape LOD and normal map replacements.
Then we have the great UL landscape series Entius Gorge is beaut.
You might add Koldorns Cave textures as well.
For a bit of fun and originality try some of JQ's stuff (I only just discovered these), and the nude spell (I don't recall the author)
is fun too.
For effects try Night Lights, New Nebular, and 3D moons.
Slof's horses are beautiful.
For houses the IC Shack upgrade - cheap and so handy to the city. House on the Bluff is in a great scenic location.
Then we come to the bodies Exnem, and Exnem variations for the female and Robert for the male are my favs.
Clothing and armour - Where do I start ? You just have to look and look some more and find what you like.
Also go to Tweak Guides.com for a good tweaking guide, and maybe try the Streamline mod.
And that's just for starters....