Sharpshooter ENB is the big one:
http://www.nexusmods.com/skyrim/mods/15105/?
(Not the easiest install ever, but not bad if you follow the instructions) Sexiest ENB I've tried. Great sun rays, great colors, great DOF, and fantastic contrast. I would say it's cinematic.
Realistic Flora:
http://www.nexusmods.com/skyrim/mods/141/? (I chose regular) This one will hurt your framerate if you're machine is lacking. But it's beautiful, especially around White Run.
Upgraded Riverwood:
http://www.nexusmods.com/skyrim/mods/18952/?
Adds quite a lot of trees, makes it feel so much more alive. Very much recommend.
Upgraded Solitude:
http://www.nexusmods.com/skyrim/mods/21148/?
Adds more trees into the city. Not as much as Riverwood, but makes the city feel more lush. I like it.
They also have white run and a few others, but it didn't make sense to have a bunch of trees in white run I didn't think, and I don't care for the rest of the cities. I might do Riften.
Enhanced Lighting and FX:
http://www.nexusmods.com/skyrim/mods/27043/?
Warning, this makes night time DARK. Most dungeon crawls are pitch black. It is intense. I'm playing as a Khajit, so the night eye power is actually a considerable asset, something you couldn't really claim in the vanilla skyrim. It also makes sneaking more believable because the world is much darker (and realistic, if you are into that sorta thing) overall. Sometimes it can be annoyingly dark in the middle of the day (if there is a storm or something) and most interiors are poorly lit...but then they would be because it's freakin' candles.
I really don't feel I need to do any more modding. The stock characters look damn good, and there is such a vast array of weapons and armor in the stock game, that I don't feel a need to expand it. The ENB and Enhanced Lights + FX really take the immersion up a notch. Night encounters without a torch or night eye or light spell are very spooky (you can see a Drauger's eyes in the dark, creepy!) but also very rewarding for a sneak type character, which I invariably choose. Overall, it makes Skyrim highly enjoyable, feels like a new game with just these few mods.